By default OpenACS 4.5 will make use of all database pools defined in your AOLserver startup script. It will demand that they all be either Oracle or PG.
However, if you add a section like this:
ns_section "ns/server/${servername}/acs/database"
ns_param availablepool pool1
ns_param availablepool pool2
ns_param availablepool pool3
OpenACS 4.5 will only use those pools and ignore others. You can then access your Oracle pool explicitly using the old "ns_db" interface, i.e. "set db [ns_db gethandle oracle_pool]".
I've done this when migrating data from openacs.org to our test OpenACS 4 website's forums, from openacs.org (which is an OpenACS 3.2, i.e. PG, site) to OpenACS 4.5/PG and OpenACS 4.5/Oracle alike.
