Getting the type Oid in a CREATE TYPE output function ..

I'm trying to create a few new types, and based on the type in/out
functions will operate a bit differently.

For the input function finding the type Oid is easy -

Oid our_type_oid = PG_GETARG_OID(1);

For output though I'm having difficulty finding out the type Oid.

I've tried using getBaseType, get_rel_type_id, and get_typ_typrelid.

Maybe I'm using the options wrong?
Or not looking in the right place?

I'm only interested in getting it working on 8.1 or greater, so if it
changes in older versions I'm not as concerned.

Any help is appreciated.

Weslee