Firstly, this is still a primarily UU team
Whimsicott doesn't need Spe EVs, stick them in HP. Its best set for OU is probably:
Whimsicott @ Leftovers
EVs: 252 HP / 252 Def / 4 SpD (SpD is Special Defense, not Speed)
- Leech Seed
It's pretty simple. Taunt and Encore are amazing together and both get priority, which is awesome. Leech Seed also gets priority and lets you help out your team. U-turn is its same old great self and the reason you use Impish over Bold. The EVs maximize bulk. You don't need more Spe with Prankster.
I strongly recommend you replace Swampert with Gastrodon.
Gastrodon @ Leftovers
Trait: Storm Drain
EVs: 252 HP / 4 SpA / 252 SpD
Pretty standard wall set here. EQ is typical awesomeness with great power, accuracy and STAB. Scald has that burn chance, great on Gastrodon as it already has awesome Special bulk. Toxic for status damage, which is always great. Recover should be pretty self-explanatory, healing off damage alongside Leftovers. Storm Drain gives you a Water immunity (yes, IMMUNITY) and +1 SpA if you take a Water move. You can opt to run Earth Power over Earthquake to capitalize more on that, and if you choose to do so you could run Calm over Sassy, as the Atk drop would be meaningless then.
Rampardos is not viable in OU. Even with a Scarf and Tailwind support. Scizor OHKOs with Bullet Punch. Breloom OHKOs with Mach Punch. Basically all the common Priority users bar CB Dragonite OHKO it. You don't have any priority so maybe switch it to one of those? I'll list Dragonite and Scizor here, as I am not that familiar with Breloom
Dragonite @ Choice Band
EVs: 4 HP / 252 Atk / 252 Spe
- Fire Punch
This thing punches holes in existence. Extremespeed, priority with a CB boost coming off this behemoth is terrifying. Outrage and Earthquake, a lot more so. Fire Punch smashes Ferrothorn, Forretress and Skarmory. Outrage is your main attack. Earthquake handles most Steels, bar the three I mentioned. Fire Punch ruins them. Extremespeed is great and allows for revenge killing.
Scizor @ Choice Band
EVs: 4 HP / 252 Atk / 252 Spe
- Bullet Punch
This Scizor doesn't quite have Dragonite's power or speed but its Bullet Punch is even more dangerous than Dragonite's Extremespeed once Technician is factored in. Superpower hits Steels that resist your priority, U-Turn is standard on a Choice set, letting you damage and switch simultaneously (it also gets STAB. If only it were 10 points weaker....) I like Pursuit in the last slot for trapping things and picking off weakened Pokes that try to run. EVs are pretty much standard fare, the extra 4 in HP because it does more for you than sticking them in either defense would. Spe is for when you aren't using Bullet Punch.
Sigilyph is solid, I've run the same set on a Sand team. It IS UU, but it can do fine in OU. Speaking from experience here. Be careful of phazers, especially Dark-type (Tyranitar) though. They CAN stop it, and they will. They will wreck this thing. So would the occasional Red Card Forretress, provided it was at full HP.
Chansey, also good. Pretty basic wall Chansey here.
What's up with Ferrothorn's EVs? And I understand using Curse and Gyro Ball, although I prefer to run Thunder Wave/Leech Seed and Power Whip myself since I switch Ferro all the time, like when said Volcarona comes in. Rocky Helmet may seem like a good idea but you're much better off with Leftovers. Ferrothorn is a defensive wall first and foremost.
Ferrothorn @ Leftovers
Trait: Iron Barbs
EVs: 252 HP / 88 Def / 168 SpD
- Stealth Rock
- Leech Seed
- Power Whip
This has a lot of staying power and when you factor in passive recovery and Leech Seed it won't be going any time soon. Power Whip gets good coverage, particularly against Water-types and it has stable power. And it lets you switch freely without worrying about losing Curse boosts. Iron Barbs does passive damage and gets fantastic synergy with Leftovers. Impish Nature drops your SpA, which you don't need anyway. The Def boost is great though, Ferro greatly appreciates it. Just keep it away from Fire-types.
"Whether Castform the Weather Pokemon can weather the weather is dependent on whether the weather that it's weathering is the proper weather for it to weather. Whether the opponent can weather the weather is also subject to whether the weather that it's weathering is the right weather to weather. Whether Castform can weather the opponent is once again dictated by the weather and whether that weather is the favorable weather for weathering. But most importantly, you should know whether there will be weather to weather rather than what weather they will be weathering and whether they can weather that weather."
- Smogon's Castform analysis