Sam, I agree with your question to Anne. If you are not talking about a Sr. Bahn Card, please provide us a link. Besides the problem with the Bahn Card, that you have to amortize the discounts against the cost of the card, it is also an abonnament, a subscription, and renews each year unless you submit a written request to cancel the subscription. Beware.
I see most routes from Osnabrück to Munster to be via Hannover or Bremen.
As is true with most Americans, the OP does not realize that u and ü are entirely different vowels. Note he spells it Osnabruck, not Osnabrück. He might as well have spelled Münster, Minster. For the record, and I am sure Sam knows this, ü is a gerundete vorderzungen Vokale, a vowel pronounced with the tongue forward in the mouth, like an 'e', and the lips rounded, like a 'u'. It is not a 'u'. See here (my mentor).
I would have assumed he was going from Osnabrück to Münster, then back to Osnabrück, then to Köln, via Münster, but it begs the question, why not just go from Osnabrück to Münster, then to Köln? That's why I thought it might really mean Munster.
Either way, he is better off using regional passes vs a rail pass.