All the charts I've seen over the years give it as a size-and-a-half difference, so a women's 11 would ostensibly be a men's 9.5. But like you said, depending on fit... If there is no men's boot with the low-rise upper, we could maybe take this to mean they run large -- up to a half size -- or maybe they squished Oscar into them a la Peter Cushing and his riding boots in ANH. Depending on how they're built, I can squeeze reasonably comfortably into a full size smaller than my ideal.
My real point was: Given the variables involved, Oscar definitely could be wearing the women's boot in that and other images where he appears to have the low-rise version, and there could easily not be a men's equivalent out there to keep looking for.
--Jonah