I think even osage with rings like that, is probably more dense than oak. So I would say that it would make decent belly slats for a backed bow. I've used some less than great stuff for backed bows and they've turned out well enough. I don't think I'd widen things much, I personally think the recommended 1.25" width has some built-in safety so you can use a range of wood quality and get what you want. I generally go a couple inches longer if I want a better safety factor, rather than wider. With a wide backed bow you run into the problem of a very thin limb, which brings with it a very small thickness taper that is really touchy when it comes to getting it to bend evenly.