Off hours/extra times for resources don’t automatically sync across between different projects since it’s possible a resource may have a different schedule for different projects they may be working on. It is possible, however, to set the same off hours for a resource across multiple projects by subscribing to a calendar where you set schedule their off hours. You can subscribe to single calendar to set off hours for all your resources, or you can subscribe to individual calendars to specify different off hours for each resource independently.
First, you’ll need to have a calendar with events scheduled during the off hours you want for your resource(s). Then each of your projects go to Project > Configure Publishing & Subscriptions… in the menu bar. Click the + button, and select either “offtime/overtime from iCal calendar” or "“offtime/overtime from web calendar”:
You then just need to choose the calendar you want to subscribe to, select whether it will be used for the project (all resources) or a specific resource, and pick if the calendar will determine Off Hours or Extra Hours:
You can now make changes to this calendar to add or remove Off Hours and these changes should apply across all of your OmniPlan documents. Hope this helps!