Olympic plates have 2" holes--actually 2-1/8" typically, to give them room to slip onto a 2" diameter bar. There's some variation in size of the holes in the plates and in the diameter of the bar, depending on whether the manufacturer is thinking in ISO/standard units (2" diameter bar) or metric (5cm, a little smaller). A lot of Olympic plates say "standard" on them. Ignore this, it's only the hole size that matters, no matter how many people try to convince a weightlifter that "size doesn't matter."
compare standard plate