Are you growing tiny oak trees in your yard?  If you have ever noticed these little sprouts shooting up and been concerned everything is going to be ok!  Oak trees have both male and female reproductive capabilities and are able to reproduce on their own. As animals such as squirrels feed on the acorns and drop them or as they naturally fall, some will germinate and start to reproduce.  This is what gives you the small shoots with the full size leaf. Typically you will just notice them in the drip line area of the tree. That is the area where the canopy reaches out. As you start to mow the lawn they will go away because they can not handle the stress.  If you can stand them being there options would be to remove the shoots by hand but that just seems time consuming. You could also use something like Roundup but that is going to kill the grass as well. The best option is just mow like normal and it’s handled. Hope this helps!