I owned both the OK3s and the PK3s, and there is one difference, apart from sound (OK3s are a tad more balanced, but cost more too). The OK3s are a little less tapered by design (go to head-direct.com and compare the photos). As a result, the OK3s started to hurt my ears with long-term use, bumping into the inside. The PK3s didn't do that. Also, the OK3s have that dumb "canal phone conversion" aspect, which is worthless. So don't think you are getting any good deal with that idea, because it's not workable.
Anyway, just something to ponder, apart from SQ differences. I used the donut pads, but some recent new PK3 owners say the full pads are better for bass delivery. Another thing to consider, whatever model you get. If the OK3 form factor isn't a problem, then they are a little bit better bud than the PK3.