Leaves aren't added to planted tanks for the benefit of the plants, they're added if the livestock have a need for them. Shrimp especially may eat the biofilm that grows on them. Also, many fish do best and it can encourage some species to breed when the leaves release tannic acid into the water, making it turn brown.
Actually, the tannins released from decaying leaves blocks light, so it's not something I'd add if your goal is just to keep a planted tank, not keep or breed specialized livestock.
Oak leaves or any other brown leaves are added to tank primarily for CRS. They love the biofilm and it's good food for newly hatched CRS. Typically, what I do, is let the leaf sit in a matured tank for a few days, so that biofilm would develop, it'll be slightly slimy to the touch after a few days. When it's ready, I transplant the leaf to the shrimp tank, they'll go nuts for the biofilm.