Oderint dum metuant - Let them hate provided that they fear. (Seneca)
Odi et amo - I hate (her), and I love (her) (Catullus)
Odium theologicum - Theological hatred. (a special name for the hatred generated in theological disputes)

Olevm addere camino

To pour fuel on the stove adding gasoline to a fire

Olevm perdisti - You have lost oil you've wasted your time on this criticism for a misallocation of resources
Olim habeas eorum pecuniam, numquam eam reddis: prima regula quaesitus - Once you have their money, you never give it back: the 1st rule of acquisiton
Olim - Formerly
Omne ignotum pro magnifico est - We have great notions of everything unknown. (Tacitus)
Omne initium est difficile - Every beginning is difficult
Omne trium perfectum - Everything that comes in threes is perfect