XA on Oracle database

The Oracle RDBMS is XA compliant from scratch - no adjustments needed - http://www.orafaq.com/wiki/XA_FAQ is a good starting point on this subject.

One may create additional views for dealing with troublesome situations. The Oracle views v$XATRANS$ and V$PENDING_XATRANS$ may be created in order to help with diagnostics (they are created by running @?/rdbms/admin/xaview.sql).
Another very good view is accessible at http://blog.tanelpoder.com/files/scripts/gtrans.sql. I usually change the join between sys.x$k2gte2 and v$session into LEFT JOIN as still it is possible to have on a database active distributed transactions not connected to any session.

Update from 6 May 2014

Lately we have got a request to fix the cooperation between the Oracle RDBMS and the Tuxedo.
The errors as following:

080307.tux24int!TMS_ORA.18176.1.0: 05-06-2014: Tuxedo Version 11.1.1.2.0, 32-bit
080307.tux24int!TMS_ORA.18176.1.0: LIBTUX_CAT:262: INFO: Standard main starting
080307.tux24int!TMS_ORA.18176.1.0: CMDTUX_CAT:409: ERROR: xa_recover() returned -3 for group ON1LT
080307.tux24int!TMS_ORA.18176.1.0: LIBTUX_CAT:250: ERROR: tpsvrinit() failed
080307.tux24int!tmboot.18147.1.-2: CMDTUX_CAT:825: ERROR: Process TMS_ORA at tux24int failed with /T tperrno (TPESYSTEM - internal system error)
080307.tux24int!TMS_ORA.18177.1.0: 05-06-2014: Tuxedo Version 11.1.1.2.0, 32-bit
080307.tux24int!TMS_ORA.18177.1.0: LIBTUX_CAT:262: INFO: Standard main starting
080308.tux24int!TMS_ORA.18177.1.0: CMDTUX_CAT:409: ERROR: xa_recover() returned -3 for group ON1LT
080308.tux24int!TMS_ORA.18177.1.0: LIBTUX_CAT:250: ERROR: tpsvrinit() failed
080308.tux24int!tmboot.18147.1.-2: CMDTUX_CAT:825: ERROR: Process TMS_ORA at tux24int failed with /T tperrno (TPESYSTEM - internal system error)

Well, it seems that one needs some adjustments - of course this is possible it is our configuration, where the devil hides. Anyway despite the running the xaview script I also granted the privileges below to the user, which the Tuxedo connects to

grant SELECT on SYS.DBA_PENDING_TRANSACTIONS to {schema};
grant EXECUTE on SYS.DBMS_SYSTEM to {schema};
-- this one is to a view created by xaview.sql
grant SELECT on SYS.V$XATRANS$ to {schema}; 

It seems it solved the issue.