OPB tables are the basic repository tables which are created when we create a repository.These tables are continously updated whenever we make a change in our repository like creating new objects,modifying existing objects,deleting objects.
REP are the views not the tables.These are the views which are created on our OPB tables for querying purposes.
Since any unauthorised modification in our OPB tables and REP views can make our repository non-workable,their acess is very limited.we even cant SELECT query on them.