If you're holding OPEN.BTC, you can withdraw it into a Bitcoin wallet through OpenLedger's Gateway service. If you're holding BTS, bitBTC, or something else on BitShares you can either trade for OPEN.BTC first and then withdraw them, or you can use the Blocktrades.us Bridge service to convert and transfer in one step. Both Bridges and Gateways are available through the Deposit/Withdraw button at the top of the BitShares wallet. There are a few other Bridge services available elsewhere also, like Shapeshift.io and Changelly.com.