Fetch your extender leaf. Ideally, it should be stored in a cool, dry area rather than in a damp basement.
Take a close look at the sides of the leaf. You should find pegs extruding from one side and holes for pegs on the other.
Draw one end of the table toward you until the table top opens at the middle. Depending on how your oak table is fabricated, you might need to pry the table apart by pulling on both ends. In that case, you will need help from another person.
Lay the leaf into the center interval so its pegs face the peg holes in the table.
Thrust the leaf pegs into the peg holes of the table and vice versa.
Squeeze the table halves toward the leaf to firmly shut the table.