Manual:$wgDBOracleDRCP

Details
Set true to enable Oracle DCRP (supported from 11gR1 onward). To use this feature, set to true and use a datasource defined as  (i.e. in   definition set   in   block).

Starting from 11gR1 you can use DCRP (Database Resident Connection Pool) that maintains established sessions and reuses them on new connections.

Not completely tested, but it should fall back on normal connection in case the pool is full or the datasource is not configured as pooled.

And the other way around; using  on a non pooled datasource should produce a normal connection. When it comes to frequent shortlived DB connections like with MW Oracle tends to mess up. The problem is the driver connects to the database reasonably fast, but establishing a session takes time and resources. MW does not rely on session state (as it does not use features such as package variables) so establishing a valid session is in this case an unwanted overhead that just slows things down.