<p>I am coming from the SQL server world where we had uniqueidentifier. Is there an equivalent in oracle? This column will be frequently queried so performance is the key.</p>

<p>I am generating the GUID in .Net and will be passing it to Oracle. For a couple reasons it cannot be generated by oracle so I cannot use sequence. </p>

There is no uniqueidentifier in Oracle.
You can implement one yourself by using RAW (kind of a pain) or CHAR.  Performance on queries that JOIN on a CHAR field will suffer (maybe as much as 40%) in comparison with using an integer.
If you're doing distributed/replicated databases, the performance hit is worth it.  Otherwise, just use an integer.