Although I haven't beaten the game yet, I've found a great way to farm Overcharge for those that are a little OCD and need to buy everything in the game. It's also a great place to level weapons and make some cash on the side. On the mission, A Hero's Duty, after protecting the Bushido Troop and making your way over to help Sam you'll come across a red bridge that will constantly have FizzCo bots and OD fighting.
Simply grind back and forth along the bridge constantly killing enemies in a glorious fashion with the weapons you wish to level and they'll be done. Since FizzCo bots drop a considerable amount of OC upon death, use the Mine all Mine amp on your favorite/most destructive weapon to net about 1000-1500 OC in 5-10 minutes. I personally used the Hair Bomb Spray, but the a fully upgraded TNTeddy works because of it's increased radius and ammo count. Since the bridge itself is so narrow it doesn't allow for much enemy movement so pretty much any weapon combo works.
I should also note that the 2 health over drives maxed out are very helpful during this sequence as you WILL be taking some damage.