Hi @xxy, how have you been?
Do you mean you have an OptionSet field in your Opportunity entity and another one in your Account entity and you want to set the value in the Opportunity form and maintain automatically the value also in the Account entity?
If that is the case, you have different approach:
From the Client: using JS you can read the OptionSet value and consuming the CRM API you can set the value in the Account entity
From the Server: you can create a Workflow to update the linked Account based on the selected value. I'll suggest you in this case to the OptionSet be a global OptionSet, so all the values will be the same no matter the field created in the entities.
