How can I resolve OID's in SQL?

Actually, the db_serial and db_user classes are already "joined". If you want to find something from db_user you can just write:

 SELECT name, owner.id as user_id, owner.name as username from db_serial;

Since the owner column in db_serial is an object, you can directly access it's members.

OID's are not directly exposed in the SQL syntax but you can use them in JDBC:

ResultSet rs = stmt.executeQuery("SELECT tbl FROM tbl"); // this gives you the OID of each row in tbl
CUBRIDOID oid = (CUBRIDOID) rs.getObject(1); 

// use the oid in another query:
PreparedStatement pstmt = connection.prepareQuery("SELECT * FROM tbl WHERE tbl = ?");
pstmt.setObject(1, oid); //this will point to the actual location on disk of the record, this will not perform a search
ResultSet rs = pstmt.executeQuery();