A 5-foot-11, 190-pound receiver out of Los Alamitos High School in Los Alamitos, California, Moore is the No. 1 wideout overall.
He had been committed to Oklahoma since September, but he is now opening up his recruitment.
Nelson is a five-star recruit, ranked No. 3 overall, and he flipped his commitment to Southern California after decommitting from Oklahoma to follow former Sooners coach Lincoln Riley. Lemon, who is ranked No. 21 overall as the fourth-best wideout, also flipped to the Trojans after Riley left Oklahoma.
At one point, the Sooners had three ESPN Junior 300 wide receivers committed with Moore, Lemon and Brandon Inniss, who decommitted in November and has yet to announce a new destination.
The Trojans are in play for Moore, as is Louisville and a handful of other programs. Now that Moore has decommitted, additional schools will likely try to get in the conversation.