Slide the utility knife blade into the split in the cutting board. Carefully spread the split open with the blade so that it is open enough to allow you to squeeze glue into it.
Squeeze wood glue into the split in the cutting board and remove the utility knife blade. Press the split together with your hands.
Clamp the cutting board tightly to squeeze the split together using a wood clamp. Allow the glue to dry for 24 hours and remove the clamp.
Sand the glued area with a fine-grit sanding block until the repaired area is flush with the rest of the cutting board. Always sand with the wood grain. Wipe off the sanding dust with a tack cloth.
Rub cutting board oil over the cutting board surface to replenish the finish on it using a cloth rag.