If you want to understand the difference between these two throws, you must know the translation first, O goshi is often translated to “full-hip throw” while “Uki goshi” is often translated to “half-hip throw”.
This is the first and biggest difference, when you execute O goshi, you have to pivot 180° and lower your level until your back is fully glued to your opponent’s belly. Obviously the entry has to be with the side of the hip, but once the toss is initiated, you have to be looking at the same direction your opponent is facing.