Question : What happens to the OLD .OST file(s) (do I need them anymore?

I have a client using Outlook 2007 as his mail client.  The only configured email account is for an Exchange account.  It has the option set to cache files.

Currently, his .OST is just over 2 GB in file size.  The size is something like 2.4 GB I think.

While troubleshooting some other variables in Outlook, I decided to delete and re-create his email profile (inside of control panel).
When I did this, is current Outlook.ost file was re-named to Outlook0.ost.  A new Outlook.ost file was created.

After all the folders inside of Outlook were 'up to date' with the Exchange server, the new Outlook.ost file was about the same size (2.x GB).  The 'old' existing Outlook.ost file again was re-named to Outlook0.ost.

I was wondering if there was any need to keep that file since he is working fine again.

I've seen times in the past when the Outlook.ost files get stacked up in the folder (outlook1.ost; outlook2.ost, etc, etc.)

Any thoughts?

Answer : What happens to the OLD .OST file(s) (do I need them anymore?

Once you delete the Outlook Exchange profile, that associated .OST becomes orphaned and should be deleted. It had a security association with the old profile, this is why you see them stack up as you have seen.

HTH.