If a person is over the hill, they have reached an age when they can no longer perform as well as before. Gran! You say you're over the hill, but you're a super cook!
over the moon
If you are over the moon, you are absolutely delighted. We were all over the moon when we heard the good news.
over the top (OTT)
Something which is over the top is totally excessive or not suitable for the occasion. Her dramatic speech was way over the top.
overplay your hand
If you overplay your hand, you are overconfident and spoil your chances of success by trying to obtain too much. Sam is hoping for a bonus for his good results, but he may be overplaying his hand if he asks for a promotion.
overshoot the mark
If you make a mistake as a result of misjudging something (situation, distance, amount, etc.), you overshoot the mark. He overshot the mark by about 20%.
overstep the mark
If you overstep the mark, you go too far and upset someone by saying something or behaving in a way that is unacceptable. Jenny is angry with her son. He overstepped the mark when he called his grandfather an 'old fool'.
close as an oyster
Someone who is as close as an oyster will never reveal something told in confidence or betray a secret. Sophie will never repeat what you tell her. She's as close as an oyster.
the world is your oyster
This expression means that you are free and able to enjoy the pleasures and opportunities that life has to offer. She left college feeling that the world was her oyster.