I hadn't any previous experiences linked with with OkCoin exchange but it seems they are the biggest exchange bitcoin market currently operating. It is unlikely that they are trying to scam you. But again MtGox was the biggest as well. And I checked theirs site and I personally wouldn't trust a company which couldn't even code their own FAQ (in english) page:
https://www.okcoin.com/t-13.htmlEDIT: to p2pbucks post below: What a great system they have then...