Hi Jonny,
Sounds like a great project. Essentially, the OM is a 14 frets to the body instrument with a 25.4" scale. The OOO originally had a 24.9" scale, and was 12 frets to the body. The body was also longer, but when it was converted to a 14 fret instrument, they shortened the upper bout and increased the scale length a bit. I think their may be a difference in the nut width too, but I'm not sure on that. Check the Martin site for original specs. I think they're on there somewhere. The UMGF might also have the original dimensions listed somewhere as well.
Sapele is a fine wood for a first guitar. Kits from LMI or StewMac, or any of the other kit vendors here will be a great way to start. LMI certainly has a really nice kit Wizard which allows you to pick almost any combination of woods and appointments.
Best of luck on your project! Keep us updated with pictures of your guitar in progress and also of you and your dad working on it if you're comfortable with that (personally, I hate having my picture taken at all, let alone posting it on the internet where it gives guys like our friends Hesh and Billy opportunity to work their *magic* on it. Stick around, you'll see what I mean at some point on this web site...).
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