BitMEX also has a weekly rebalancing for all their contracts, but currently their most popular product is the Daily 100x maximum leverage contract. This settles daily, but otherwise the rest of the contracts are handled on Friday, a little bit after OKCoin's 8:00AM UTC. See this website to get a countdown to the settlements on the different exchanges. The policies of exchanges are changing often so this information may be outdated by the time you read it (though we will try to keep it as up to date as possible).
In this guide we don't want to deal with social loss risk, so use the FCA-regulated, London-based bitcoin derivatives exchange CryptoFacilities. They offer contracts with 2% margin requirement (50x leverage) as well as a 15% margin requirement (6.5x leverage). They are 100% bitcoin based, but they don't accept US customers. Sign up here to get started: