General:
* overboard: (adverb) excessively; to an extreme degree.
* overcast: (adjective) cloudy; covered with clouds.
* overcome: (verb) to succeed in dealing with a problem or difficulty.
* overdue: (adjective) past the expected time; late.
* overjoyed: (adjective) very happy; delighted.
* overlook: (verb) to fail to notice something; to miss seeing something.
* overreact: (verb) to react in a way that is too strong or extreme.
* oversleep: (verb) to sleep longer than intended.
* overwhelmed: (adjective) feeling as if you are going to be defeated by something difficult.
More specific:
* overabundant: (adjective) more than enough.
* overambitious: (adjective) having goals that are too difficult to achieve.
* overanalyze: (verb) to think too much about something.
* overbearing: (adjective) bossy and domineering.
* overconfident: (adjective) too sure of yourself.
* overestimate: (verb) to think that something is bigger, better, or more important than it really is.
* overindulge: (verb) to allow yourself to have too much of something enjoyable.
* overpopulated: (adjective) having too many people living in a particular place.
* overprotective: (adjective) too concerned about the safety of someone, especially a child.
* oversimplify: (verb) to make something seem simpler than it really is.
* overt: (adjective) done or shown openly and not hidden.
Phrases:
* over and over: repeatedly.
* over the moon: extremely happy.
* over the top: excessive; too much.
* over with: finished; completed.
This is not an exhaustive list, but it gives you a good starting point.