Perhaps it's due to the childhood influence of AA Milne and the "wolery," but I thought owls nested in enclosed spaces such as hollows in trees. Would they be at all interested in a platform for nesting?
That's a really good question. I think Great Horned Owls normally prefer to nest in trees, but they do seem to like peregrine nest boxes too. I think a platform might be too much out in the open for them, but even if they only like to perch there, it would deter the ospreys from using the nest.
A couple of different GHO nests: last year, a Great Horned Owl raised an owlet in a planter in Brevard County, Florida:
And in Socorro, New Mexico, a pair of GHOs raised their owlets in the fork of a tree, which had been provided with a liner. They had tried to nest their unsuccessfully the previous year.