Hi Cymro,
Oita is very limited as far as hotels go.
Fukuoka would be the best option but remember that all those other fans attending the quarter finals have got the same problem. So the trains are going to be packed.
I agree with the comment "this years Cardiff" from David.
I remember hearing from friends who were queued at Newport and full trains going through and not stopping.
2 sites to check with
www.hyperdia.com
https://www.rome2rio.com/map/Kumamoto/Ōita
I've ended up booking an APA hotel which is cancellable in Kitakyushu as mentioned previously. Will cancel if anything closer pops up over the next year, but I doubt it and even if it does, I'm sure it will be extortionate, so I can see us sticking with this option as it's a very reasonable price and right next to the train station.
As the stadium capacity is 30,000 less than Cardiff, and Japanese transportation infrastructure seemingly far superior than in South Wales, I hope there'll be no nightmare stories of getting to and from for those commuting in! The downer is we may have to miss the quarter final matches that are on afterwards to make sure we get on a train back.