Is this a walnut?

The quick answer is: no. It's a growth on a limb of an oak tree called a gall. It is made by a tiny wasp called a gall wasp.

I found this gall on a dead limb on Sunday, Feb. 11. There was not an exit hole anywhere on the gall. I knew that the gall should have a small white grub inside, but I wondered if it would be alive since the limb was dead. So I cut the gall open to see.

Inside the outer shell of the gall is a brittle case. I carefully broke open the case with the point of my pocketknife. I was surprised to see a living white grub inside the case. It will probably die now that its protection from rain, freezing temperatures and predatory birds has been breached. I am sorry for that, but I wanted to know so I could tell you. Gall wasps are common insects although they are not often seen.