Although these orbs may look like a fruit or nut (and are found nestled amongst fallen acorns) they are neither. These are oak apple galls. They are formed when a gall wasp injects her egg into the mid-vein of a budding oak leaf. The wasp not only injects her egg, but also a combination of chemicals which cause the tree to form a protective gall around the insect egg. The larva will develop inside this gall and eventually emerge when it's reached maturity. On the smallest of the three light brown galls pictured here, you can see a small, round circle which is most likely where the adult wasp exited the gall.
I have three different-looking oak apple galls pictured here, and I wish I could tell you why they are all different, but frankly, I don't know. The light brown ones are clearly galls that have reached maturity and served their purpose (as the exit hole indicates). The smaller, darker red galls are solid and when squashed release a red juice. I've dissected these in the past and have found no evidence of an larva inside so perhaps these are galls that don't form correctly. I have read that the red juice is high in tannins and can be used as paint or stain.
The smallest, bright green galls seem to be galls that were knocked down prior to reaching maturity. The culprit could have been wind or over-eager squirrels who spend these months frantically collecting as many acorns as they can, and in so doing litter the ground with acorns and galls. As kids we used to call them "Frogs Brains." I have no idea why.
It appears to have been a banner year for the gall wasp as I've found hundreds of galls on my walks this Fall. Wherever you find acorns, you're likely to find galls. Take some time to pick one up, break it open, and engage your youthful curiosity. It will be worth your while.