Historically Oracle OFA (Optimal Flexible Architecture) recommended to seperate data and index files. Nowadays in the age of SAN, does anyone know whether this is still valid?

This is not needed anymore. Only in very special situations it can help. General rule should be: spread I/O. This used to be done on putting things on diferent disks. but nowadays striping and mirroring are that common, that the old idea is history. Or at least it should be.

Kind regards,
Dick

I agree with Dik, nowadays the strategy is to stripe and mirror everything. One important exception : keep the archive logs apart (for security and contention). Eventually also the redologs can be separated and put on fastest disks available (with mirroring).