O/S means one size fits most...lol....yet I dont seem to be a size most♡ive seen these these fit depending on how youre built and if you like thongs n fit small to a large xlg I just couldnt say ... I hope this helps ...if youre a small med maybe large n u like the fit n type I dont know I dont wear these kinds of undies im a brief girl ... sorry if anyone else has any comments please feel free to leave them , thank you hun ♥♡♥