Ok, so first of all, this team was inspired by magic9mushroom's Smogon RMT based on the same Slowbro set, and the final version of my team ended up being pretty similar to his. Credit where credit is due.

So as the name of the RMT implies, the goal of this team is to utilize and abuse Withdraw Slowbro. It's a pretty neat set and a fantastic win condition that has the advantage of both tanking physical hits far more effectively and winning PP wars with opposing Slowbros thanks to Withdraw's enormous PP. Aside from Slowbro, the team is pretty basic and standard. As with many RBY OU teams, there's only so much you can do to improve it (honestly, you could toss on 6 OU Pokemon randomly and you'll probably have a decent RBY OU team), but there are a few different versions of this team that you could use if you want to try it out for yourself. Now for the team itself:

Alakazam @ No Item
-Thunder Wave
-Seismic Toss / Reflect

Alakazam is probably my favorite lead in RBY OU. It just makes such a great sleep absorber and it forces lead Gengar out from turn 1. It's also pretty reliable at spreading paralysis everywhere and just being a generally sturdy special tank. The moveset I'm running is pretty standard, but I do have a bit of a tossup between Seismic Toss and Reflect. Generally speaking, Seismic Toss is better in the lead position since it lets Alakazam at least do something to lead Jynx and Exeggutor. It's also kinda useful mid-game for dishing out a little damage while conserving Psychic PP. However, Reflect has an interesting relationship with lead Alakazam. Opponents tend to assume that if I'm leading with Alakazam, I'm running Seismic Toss, so they get a little more comfortable about blowing up an Exeggutor or Gengar or something on me. If I can lure them into that false sense of security and set up a Reflect before they can move, Alakazam can easily soak up an Explosion and they lose a Pokemon. It's not often that this happens, but it has occurred a couple of times.

m9m runs a Starmie here, which is a perfectly valid option. I prefer Alakazam since I find it to be a bit better of a sleep absorber and special tank in general. It also switches into Gengar and the occasional Victreebel more easily, both of which give Slowbro a lot of trouble (especially since I'm running Surf > Psychic). I might prefer Starmie if I had Slowbro problems, but since my own Slowbro easily PP stalls other Slowbros, I'm pretty free to stick with Alakazam.

Exeggutor @ No Item
-Sleep Powder
-Stun Spore

There's not too much to say about Exeggutor. It's just a good all-purpose Pokemon that sort of holds a lot of teams together. It sleeps, paralyzes, tanks hits pretty well, dishes them out, and booms on things. Every bit of support it offers is greatly appreciated, and being able to blow up and severely cripple or outright KO a Zapdos or Chansey or something is great for Slowbro sweeps. That's about all I have to say about it, pretty standard stuff.

Chansey @ No Item
-Thunder Wave

RBY Chansey is the special tank to end all special tanks. It stops Starmie 100% of the time outside of ridiculous hax, it paralyzes a ton of stuff, and it just stops a lot of special attackers from doing much of anything (be careful of boomers, though). Like m9m, I'm running Blizzard > Ice Beam since I'd honestly rather paralyze opposing Chanseys than go for the freeze gamble. That sounds weird, but a paralyzed Chansey can be beaten much more easily by Slowbro than an unstatused Chansey if I can get a well timed full paralysis. Frozen Chansey is even easier to beat, but freezing Chansey isn't something to rely on, especially if it means risking a freeze on my own Chansey. Paralyzing it is just a much more reliable way of making it easier for Slowbro to beat. Besides, the extra 26% more power from Blizzard is pretty much always going to be useful. For instance, it guarantees the 3HKO against Zapdos and Exeggutor and usually 3HKOs Tauros, none of which Ice Beam can do.

Rhydon @ No Item
-Rock Slide
-Body Slam
-Substitute / Rest

In my opinion, every team that carries a Slowbro of any kind needs to have one of the Rocks. Zapdos (and Jolteon, for that matter) is pretty scary for Slowbro since it has a powerful STAB Thunderbolt and a fairly high critical hit ratio to bypass Amnesia. The Rocks are the best answer to Zapdos out there. I prefer Rhydon due to its greater power and bulk, but Golem is usable. However, the main perk of Golem is the ability to blow up on things, which you'd want to avoid anyway if you're relying on your Rock for Zapdos. Still, blowing up on Starmie is great against non-Zapdos teams. At any rate, the set I'm running is standard, except I've been messing around with Rest in the last slot. Substitute is decent, but it's pretty situational and close to useless if the opponent's sleeper is still alive and nothing else on my team has taken the sleep (or has but woke up). Plus, I need to be careful about throwing away 25% of my health when Rhydon is so important for Zapdos. Rest is pretty cool, though. It lets me play a little more recklessly with Rhydon in case I want to switch it into random Normal attacks, Gengar, or whatever else since I can just heal it up later. It also helps against random gimmicky Toxic Zapdos, which actually has a shot at beating Rhydon if the opponent predicts switches well and gets lucky with crits. Besides, a sleeping Rhydon walls Zapdos just as hard as an awake one.

Snorlax @ No Item
-Body Slam
-Hyper Beam

I honestly think that every RBY OU team needs to have at least one of the physical Normals: Snorlax and Tauros. I've tried Tauros out in this spot, but I picked Snorlax > Tauros in the end for a few reasons. Due to this team having paralysis everywhere, the extra speed of Tauros is less necessary, so Snorlax's extra bulk comes in handy for checking a lot of things. The extra bulk also allows Snorlax to survive longer and paralyze more opponents, which Slowbro likes. The greater power helps in general as well and lets Snorlax put more pressure on opponents. Every bit of damage helps; the weaker the opponent's team is, the less Surf PP Slowbro has to use to KO opponents. Blowing up on things is also great since it essentially guarantees a kill against anything not named Gengar, Rhydon, or Golem. I also prefer Snorlax because it lures in Gengar a little more easily than Tauros since Gengar might be expecting the boom or might think I'm running Surf > Earthquake, so Snorlax can sometimes dispose of it that way.

Slowbro @ No Item

This is Withdraw Slowbro, one of the most unique and fun sets that I've ever used in RBY OU. As I mentioned before, Withdraw has two big perks: it lets Slowbro tank physical hits even more easily, and it lets me PP stall opposing Slowbros and win 1-on-1 against the standard Tobybro 100% of the time. I've chosen to run Surf > Psychic for a couple of reasons. The biggest one is the extra PP, which helps not only prevents Slowbro from running out of STAB PP when trying to sweep (this has happened once with Psychic), but it also lets Slowbro win the PP war against opposing Withdraw Slowbros that run Psychic (this has also happened, lol). Surf also helps against Alakazam, who is easily 2HKOed by Surf at +4 but would force me to burn a lot of Psychic PP to beat it even at +6 (and I still need a Special drop to 2HKO).

The trick to using Withdraw Slowbro is knowing when enough is enough when boosting. It's tempting to get greedy with boosts, but generally you should aim for a couple of specific benchmarks and only boost to maximum when you're absolutely sure that you have a safe opportunity to do so. Here are some benchmarks for Withdraw:

+1 Defense: Tauros can no longer KO with Body Slam + crit Body Slam + Hyper Beam. Snorlax can also not KO with 2 non-crit Body Slams and a Hyper Beam or Selfdestruct.
+2 Defense: Snorlax can no longer KO with 3 Body Slams and a Selfdestruct, in case you suffer full paralysis during one of those turns.
+3 Defense: Snorlax can no longer KO with Body Slam + crit Body Slam + Selfdestruct. It can still win if it gets a full paralysis and gets 2 Body Slams + crit Body Slam + Selfdestruct, however.
+4 Defense: Now Snorlax has an extremely small chance of KOing with the aforementioned combination. This is as high as you should go until you're very confident about boosting to max. At this point, you're at 954 Def, which is only 45 points away from the 999 cap, so the next boost won't help much anyway.
+5 Def: Now you're at the 999 cap, which is as high as you can go. Snorlax now can no longer KO with the aforementioned combination and will need extreme hax to win (as will non-Thunderbolt Tauros). The only time you should go to +6 is if you're trying to PP stall with Withdraw.

Now for Amnesia:

+2 Special: Zapdos and Jolteon's Thunderbolts fail to 2HKO, Gengar's fails to 3HKO, and Starmie, Lapras, and Chansey's fail to 4HKO. Persian's Thunderbolt + crit Thunderbolt also cannot KO. Likewise, the occasional Thunderbolt Tauros cannot KO with Thunderbolt + crit Thunderbolt either.
+4 Special: Zapdos cannot 3HKO with Thunderbolt, and Jolteon cannot even 4HKO. Tauros only has a very tiny chance to KO with two non-crit Thunderbolts + crit Thunderbolt, and Persian fails altogether.
+6 Special: Zapdos only has a shot to 5HKO with Thunderbolt, as does Jolteon (barely). All weaker Thunderbolts fail to 5HKO outside of hax. That's all I can think of as far as benchmarks go, but this is maximum you can go.

The main thing to remember with these benchmarks is that you want to hit one so that you can survive for 4 turns at a time. You need 3 turns to be able to use Rest effectively, but then you risk getting caught in a Rest loop until either the opponent runs out of PP or bad hax happens. If you can survive 4 turns, then you can have those 3 turns for Rest and still have an extra turn to do something productive and slowly gain positive ground. Also, keep in mind that critical hits are a commonplace in RBY OU and can mess up pretty much all of these calculations. A critical hit Thunderbolt from Zapdos or Jolteon will be a guaranteed OHKO, and one from Gengar will usually KO. One from Lapras, Starmie, or Chansey can deal a damage maximum in the 90s, so they are still pretty dangerous. Needless to say, if you can't remove strong Thunderbolt users before attempting to sweep, then there's a good chance you'll be playing Russian Roulette with Slowbro.

That's about it for the team, though. Despite how much I've written in this RMT, the team itself is actually pretty simple and really fun to use. I've been playing on both the PO and PS! RBY OU ladders for a long time now, and I'm almost undefeated with this team (which is probably more of a testament to the skill of people on the ladder, but I digress). Try it for yourself, maybe swap a few things around and make your own version, and if you see something about this version that you think could be improved on, rate away!