Ironically, one of Charizard's best teammates in OU is actually the least popular Kanto starter, Bulbasaur. They both function best under the sunlight, and Venusaur can help remove bulky water types early in the game to help clear the way for a choice scarf, solar power Fire Blast. Exeggutor and Sawsbuck can do this as well.
Getting rid of Heatran is also extremely important, so Dugtrio could be an option. It would also help fight the weather war to make sure Solar Power is ready.
Ninetales is a must in OU. SubstituteBellyZard faces too much competition with Darmanitan now in days. The only thing it's really got over the ape is just a little bit of speed, but it has much less power. So, really, you're best just going for Solar Power if you're going to use it in the tier at all.
And, obviously, the dragon's health is going to be lowered quite a bit by solar power and stealth rocks, so folks like Forretress and Donphan can be very helpful. Though if the writers of the POTW are any good(which I'll bet you guys are), that should really go without saying.