Some HO/N systems run on 15.4 VDC where as O seems to want 18V and G runs on 21V. A 5amp powerpack should run an engine with a can motor, no problem, though Ive not tried running O scale DCC on 15V.
I'm using the ESU Cab Control and it came with a 7A switchable voltage powerpack which seems capable of running 2 O scale locos @18V. Check the documentation of whatever system you have, some of them cannot handle more than the 15.4V HO powerpacks, and some will run the larger scale stuff.