OS authentication is not enabled by default in Oracle XE. To allow the use of OPS$ users, open sql plus (All Programs -> Oracle Database 10g Express Edition -> Run SQL Command Line) and run the following command:
alter system set os_authent_prefix=OPS$ scope=spfile;

Restart the DB:
shutdown immediate
Startup

To Create the OS user:

CREATE USER “OPS$DOMAIN\DOMAINUSER” PROFILE

“DEFAULT” IDENTIFIED EXTERNALLY DEFAULT

TABLESPACE “TABLESPACENAME”

TEMPORARY TABLESPACE “TEMPTABLESPACENAME” ACCOUNT

UNLOCK;

You can grant  the appropriate roles just like any other user

GRANT CONNECT, RESOURCE  TO

“OPS$ DOMAIN\DOMAINUSER “;