Just like in writing, choose your dominant hand in holding your pen... same thing that apply in osu tablet play, on my opinion. Although, I use both mouse and tablet with my left hand but holding the pen with your dominant hand is a recommended setup for playing with tablet for starters.
One of my left handed friends says he uses the tablet pen in his dominant hand and uses the "/" and shift keys instead of z x. Apparently trying to use his right hand was awkward or something and he had trouble gripping the pen properly as well as moving it.