I never used OKex before, and I only use Bitmex. I think it depends on what we feel on each exchange because so far, I don't have any problem for trading in Bitmex. Besides that, I don't have any experience of getting a freeze on the trading, maybe its because your internet connection gets lack or get a problem. Maybe later I will take a look at OKex, and if I can see that exchange is better than Bitmex, maybe I will use it.