I think a safe maximal drawweight for a red oak selfbow is about 65# in practice. You could make a wide and long flatbow of 75# maybe, but the wood must be flawless then. A working backing of a tension strong wood (hickory, white oak, even bamboo) may gain you another 10 pounds in weight. Rawhide may also work to some extend, but it's not as good as a wooden backing.
If you're looking for a higher drawweight, you should be looking for other wood species, preferably as staves.
With ring porous woods, such as red oak, you don't necessarily want either thick or thin growth rings. Instead, you want a lot of latewood, and very little earlywood. The earlywood is the porous, crumbly wood between the darker coloured solid wood.