Well here's some of the fish I've kept w/O's without many problems...
Severums
Tinfoils
Bichir
Silver Dollars/Red Hooks
Anostomus headstanders (but haven't seen them around in awhile)
JD's & Firemouths (but they can be a bit too rough for the O)
Geo. jurupari (very cool fish)
Redtail Chalceus (again, not sure you can find some)
big clown loaches
Now not sure some of these guys would fit in the 75 w/the O, as some of them get pretty big, but they're fish I've kept peacefully with my O's.