The width of the stock will determine the limits on the arc. A narrow piece will limit the arc.
Start with a roll of paper long or wide enough to draw out the leg. Use a stick to draw the arc. Pin it at one end and drill a small hole on the other for a pencil.
Lay the Oak on the paper and trace its edges on the paper. The arc must fit within those boundaries. If you have a pattern, use it instead. You don't mention the length of the leg, so if you don't have a pattern you're going to have to work out an arc the proper length that fits within the width of the stock. Don't forget that you likely have to cut four legs, so allow for that. Move the base of the stick back or forward to make the curve sharper or milder.
Once you draw the curve, cut it out carefully and transfer it to the stock. Then transfer 3 more. Then cut out with a band saw, slightly oversize and sand down to size and smoothness.
Without knowing dimensions, this is about as far as I can go describing it. BTW, the pin needs to be set 90 degrees to the stock, use a carpenter's square to set the 90.
Tommy McDonald last week built a chair with exactly the kind of curved leg you are building. I couldn't find it on YouTube. Aha! I found the episode on his website. Just what you're looking for.
Episode 0607: Library Chair with Tom McLaughlin - Tommy Mac