Weird. It's always been a PST file for me. Even with Outlook 2000.

The .ost file is an Offline folder store. The .pst file is a normal folder store. One is used when your primary storage is on an Exchange server. The other is used if you're using Outlook as a POP3 client.

If you use AutoArchive, you'll probably have both (outlook.ost and archive.pst).