Basically brain think u need much energy on hands for osu game. Even though u dont need to. Just practice by retrying maps that aren't that difficult but you tense up on, focus on not tensing up and keep relaxing. Play a lower difficulty to remind your brain what it means to relax.
If its your aim hand that can't relax, remind yourself that you only need to grip hard enough so that the pen stays in your hand, the rest of the energy should be put to focus on moving and controlling the pen, identify which parts of your hand controls that and focus on them.