Cook corn in a pot of boiling water until tender, 6 to 8 minutes. Transfer to a bowl of ice water and let cool. Pat corn dry, then cut kernels lengthwise from cobs into long sections. Break into large pieces.
Cut rind from watermelon and cut melon in half lengthwise. Slice crosswise about 1/4 in. thick. Slice ricotta salata about 1/8 in. thick and set aside.
In a large, wide bowl, gently combine corn, watermelon, tomatoes, and spinach. Toss very gently with dressing. Ease salad onto a platter and tuck in cheese. Scatter oregano leaves over salad and sprinkle with pepper and salt.
Make ahead: Salad, through step 2, up to 2 hours; dressing, up to 2 days, chilled.