After 17ish frames MK is "invincible" until around when he reappears. Nothing can interact with him, but I'm not sure if there is another term for what he does there.
Uair OoS is more of a combo starter. Also note that you will have a tough timer hitting shorter characters with a Uair OoS if they are on the ground. Nair and Up B are your killer OoS moves. Uair combos at almost all percents until you are ready to kill. If someone is in range to get hit by Uair any time under 100, I'd definitely go for it and follow up.
Uair OoS is more of a combo starter. Also note that you will have a tough timer hitting shorter characters with a Uair OoS if they are on the ground. Nair and Up B are your killer OoS moves. Uair combos at almost all percents until you are ready to kill. If someone is in range to get hit by Uair any time under 100, I'd definitely go for it and follow up.