Oak seedling removal
First, determine if these are saplings from acorns, or are these suckers sprouting from the root of a larger, parent tree? Dig a couple up, if they are attached to a larger main root running back to a large tree they are suckers. If you see an acorn still attached, they are saplings.
If these are sprouted from individual acorns apply brush killer or non selective herbicide to the plants, while shielding the grass and other desirable plants, to kill them. (You can use a cardboard ‘shield’ and paint herbicide on with a disposable brush) You could cut them near the ground and immediately ( within 1-10 minutes) paint the stumps with herbicide. Repeated mowing will kill them, too. Please read and follow all precautions on any chemical used.
CAUTION- If these sprouts are coming from the roots of a larger tree, the larger tree can be damaged or killed when using herbicide on these suckers. In that case the only safe way to deal with the suckers is to repeatedly mow or cut them. Mowing them is an option as long as it is done before they get too thick for the mower blades. Keep mower blades sharp. Improving the health of the parent tree may reduce the suckering because some trees will produce suckers in reaction to stress, like drought or root damage.
Here are some references for you-
I hope this helps. Thank you.