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For pure option buyers, you could always choose isolated even if you are in PM so that you don't need to worry about any margin requirement for long option positions. If you intend to use cross margin like margin offsetting in PM or stablecoins as margin, please choose cross.

If you are pure option buyers or option is just a small part of your overall portfolio to hedge or enhance yield on delta-one or spot positions, you do not have to switch to portfolio margin at all. The user guide of Portfolio margin is available here. Please contact your account manager or institutional@okx.com if you require a thorough examination on margin calculation. Please note Portfolio margin's margin calculation is based on a risk unit, or underlying concept.