There are few Oracle installations out there (as far as I know...) So I would reccomend contacting EPiServer support to get a proper answer :) http://world.episerver.com/en/Support/
I only have experience in working with Oracle databases as backend systems in my projects, but I've experienced that there are big differences between Oracle 9 and 11 both on the server side and on the client side. (ODP.NET)
Ok, I realise this is probably going to be like selling shares in BP to a fisherman in the southern US, but... is anyone running EPiServer 4.5x/4.6x with Oracle 11 ?
Officially Oracle 11 is not supported with EPiServer 4 / 5 R1 so if anyone knows of any pitfalls or problems please let me know :)