Yes, you can OC on the stock fan, but a lot of people don't recommend it. I've never tried going excessively high, but mild OCs are possible. The question becomes one of temps. The stock fan won't keep your CPU terribly cool which does have the possibility of causing stability issues, or in worst-case scenarios, CPU damage and shorter lifespan. That being said, it also depends on what kind of load you're throwing at the CPU. Prime95 will max out your CPU more than any game will, so will also drive up the temps more. It's entirely possible that you can get a stable OC for games that would be crash under Prime 95. However that's a safety buffer that most people use to make sure they NEVER have problems in their normal use.
Bottom line, if you just want a slight OC to get a little extra oomph, the stock fan will be ok, though it will be louder and hotter than you'd get from a decent aftermarket cooler. Also, if you're looking at a tower cooler, make sure it fits in your case.
And no, you can't OC your CPU on a B75 chipset, only your BCLK. A P67, Z68, Z75, or Z77 board is necessary to mess with your CPU multiplier.