newbie question: official vs custom firmware

[Danq10]

I'm a newbie and I'd like to know the differences between the official and custom firmware.
Having installed the custom version it is possible upgrade the firmware via OAT with the official version ?   

[futeko.com]

Differences are outlined in the customer firmware thread. They're not massive, but I would recommend installing custom.

Yes you can install official over custom and vice versa.