oid vs bytea in postgres
oid as a type is a 32-bit unsigned integer that stands for object id, and is used by the system as an all-purpose surrogate key to refer to different objects and sometimes rows in pg_catalog. Being the type for keys to large objects  is just one of its many uses. Other "objects" like tables, sequences, types and more are refered to by their oid.


  But I am confused about pg_largeobject table structure, why its using
  both data types for same data?
  Is postgres internally storing oids as bytea, and just returning oids
  for reference? If its working in this manner, why can't use just
  numeric type for reference instead of oid type.


It stores the contents of large objects as small bytea pages (~2000 bytes)in the pg_largeobject table, with loid as the ID or the large object, and (loid,pageno) as the unique key for that table.
The generator of new values for loid can be compared to a integer sequence, except that after wrapping around at 2^32, it can avoid conflicts with existing values. As users, we don't have to care about this, it's managed by the internals.


  If not, what is the use of oid here and is any data sitting behind the
  oid somewhere in the database, and what is the bytea column data
  about.?


The oid is used as a handle and the bytea column holds the actual binary data.