Okex is owned by Okcoin and another important point to be added is leverage.
On Okex you are able to perform leveraging while on Okcoin you are not able to do it.
The pair you are using on Okcoin is only in CYN(Chinese Yuan) while you can use another asset to pair up on Okex.

here is a full explanation and the difference between one exchange and another;
https://www.forexbrokerz.com/okex-vs-okcoin-forex-broker-comparison