Update 1: Highs comparison been revised.
Update 2: Please, do not consider stronger bass impact of PK1 as a straight advantage. PK1 bass impact may be too heavy sometime, bleeding into and warming up the midrange. If you want more bass from OK1, then you may replace donuts foams with standart foams (without hole). You may raise it even more, using
Sennheiser MX75 foams. So it is easy to tune OK1 bass response for your tastes. In contrary, you may find PK1 bass impact too strong even with donuts foams. So there are few opportunites left to manage with PK1 bass weight.