Its rift sawn, and from the bit of grain showing in the top pic it looks straight over that section. But you have to evaluate grain on all four sides and not just the wide side, especially with rift or quartersawn wood. This is because the annual rings will look fine on rift/quqartersawn wood, but if its cut at an angle to the length you'll not have continuous fibers running on the back of the bow which is a must. Oak has some very pronounced little flecks/checks running along the grain, look on the side of the board to tell whether your grain is running parallel for the length of the board or at leat each limb. It doesn't have to be perfect, especially if you choose a conservative design, but it needs to be pretty straight for you to have a high probability of success.