Short answer, you only sign/stamp a form 1 for the work you/your AMO was contracted to do. You cannot issue a form 1 for the OWC task.
Form 1, when used post maintenance, does not certify anything as SERVICEABLE!!!
Its purpose is to certify your work has been carried out to the standard required under the organisations approval and to provide for traceability for the end user. It can not be "serviceable" until the other partys complete and certify their work also.
So, the work needing to be done should be explicit, may be divided between different organisations and certified on several form 1's. Altogether the collective documents assure the operator that ALL the work has been carried out to the required standard before the aircraft is released to service with this engine fitted.
As mono says, the wording should be clear on the form 1 so the person(AMO) certifying the engine fitment knows where it came from and what you did to it
