If you'd like help finding information perhaps you could tell us how Oliver Black or Black family and Strabane/Ardstraw are supposed to fit with post office.
Click on the Blue 'More Search Options' and type Postmaster in the Occupation 'Box'
Actually, Kiltaglassan's suggestion is incorrect. You need to put
post* into the occupation box to get results for post office office, postal clerk, postmistress, postman, etc.