Hello Paul,
I agree with what Vadim said, only reinstalling the OnDemand database is not enough. With that restore you will only have the whole configuration AND the Indexes, and nothing else (meaning NO document at all).
For the documents you have to copy the Cache directory to the test server.
If you have documents in TSM, then you need to be extremely careful. Because by doing such restore you will point to your actual OD TSM repository, as the original OD instance.
Meaning if in your test, you add a new document, it won't be visible in the other environment.
Meaning if in your test, you remove a document, then in the other OD server, you will still have the link, but no document behind...
...
So you can have a mess if you are not careful, and do more than a read-only.
As long as you play with Cache directory, there are no danger, but if you have TSM connection, be really careful on what you do.
I hope I was clear!
Cheers,
Alessandro