Installing a deadbolt on a door with a circular hole for the deadbolt location?
Do you have a deadbolt?  Try putting it in.  Although the bolt is rectangular, it is slightly smaller than the one inch hole so it should fit just fine.  The facing which guides the bolt screws into the edge of the door to hold it stable.  It is possible some chiseling will be needed to make the bolt's facing flush with the door.