f you have oak trees, you've probably had to deal with saplings coming up in your lawn.
Some years they aren't so bad, and you can simply hand pull them as they come up.
Other years, however, your lawn may look like a tree farm with saplings coming up all over the place. Pulling them by hand just isn't feasible, there are too many. There are herbicides that will take care of them, but most often these saplings are in your lawn and you don't want to damage your lawn with an herbicide.
If this is the case, here is the magical fix you were looking for: mow them down!
It's as easy as that. It will likely take several mows (3-4) to kill them off, but just keep at it and they will eventually disappear!