oot3dhax with 2.5 *hax works on the N3DS and not the O3DS. I personally own an N3DS XL, O3DS XL, and O3DS. I ran into the issue after I had successfully setup oot3dhax with the latest 2.5 *hax on my main 3DS running 10.2 and then tried to use it as a first point of entry on my O3DS XL running 10.1. Some other people in the scene that I talk to have confirmed the issue. I got an old payload from a themehax 1.2 installer someone had saved and replaced the payload.bin the installer had created using the save transfer tool in the homebrew menu. After that I was able to get oot3dhax to work on my O3DS XL and I installed menuhax on it after that.At this point, I believe the people who claim it "works" just have themehax 2.5. When oot3dhax crashes, it goes back to the home menu where themehax gets triggered immediately.
Nobody on 10.2, where themehax is patched out, has claimed that oot3dhax 2.5 works for them. Meanwhile it's hit-or-miss on lower firmwares. That's just my suspicion.
Besides what I mentioned, I also had to use a 10.1 payload with the oot3dhax_installer on my main 3DS running 10.2 to get it to work. This is another known bug, although that one is with the installer, the earlier thing is with the actual exploit.