I have to say that this is exactky the kiond of thing that gives Oceania a bad reputation once in awhile.
of course it is not your fault that you were not informed.
Oceania has a way of regularly putting deadline on things and announcing in the press - but if you don't hang around the right circles you might now hear it.
There are several user groups and forums dedicated to Oceania Cruises, and I am sure the word got around there - but as you say - it is not your responsibility to track the company.
I would think you could sue the line or your TA in small claims for the difference it will cost you. A travel agent is supposed to be the liassion between you and the cruise line.
Tough on her, but then again, someone should have to pay. She would likely get it back from the cruise line.