Well, if you have a stack of publications, you are a reviewer for JAMA and there are scores of people in high places who think that you are 'the best thing since sliced bread', go for it !
Yes, there are people who have done fellowships on O1s, but it is surely nothing you can bank on.
The criteria are very similar to EB-1 EA petitions. As a USCIS officer put it to me: 'Well, it's like trying to proove to us that you can walk on water.'
My 5 cents on visas for fellowships:
-- If you have a J1 for residency, there is not too much choice
-- If you have an H1b, you run into the problem that many fellowships don't sponsor a continuation of it.
-- If you have a H1b and are able to find someone to file a labor cert for you during residency, you might be able to do your fellowship on EAD (the EAD is not limited to the prospective employer).
-- If you come only for a fellowship, the J1 is no problem if you intend to return (but who does).
-- If you come for a fellowship first and residency afterwards (yes it happens), you have to be careful in what you write on the initial J1 application bc you are only allowed one change in specialty.