Very interesting concept, hope it goes well. I've actually built Lego trains before ( https://www.flickr.com/photos/168044544@N07/ ), but I've never heard of anyone trying this - wish you luck. I expect you'll have to rebuild the trucks completely, since they're normally designed for L gauge. You might be able to carefully shorten the Lego axles by cutting them/filing them?
Okay, I checked out your Flickr link, simply saying you have built lego trains before is a glorious understatement. That 4-6-4 is beautiful! I have plans for a Switcher very similar to that B6. I don't know if I will be able to adapt a power train from lego to O gauge, so a few cars need to get built first.
I thought about cutting the axles, slipping a sleeve over the two ends and soldering them together at the right width. This still results in the wheels in wheels with a profile that just doesn't make sense for O gauge. I did some experimentation. I can make outside bearing trucks at 7 wide width. I might be able to get down to 6 and a plate using some snot techniques and technique pieces.
If I could come up with a way to mount the bearing on the inside of the axle without taking things apart that would be better, but most of the wheel sets have the axle knurled somehow to help how the wheels in position. The knurling would interfere with the bearings.