Nair is our fastest OoS option, tied with our grab at 7 frames. Well I guess grab is faster since I'm assuming it takes a few frames to start the jump. But yeah it's our fastest non-grab OoS option. It even comes out faster than Jab, which is frame 8.
The reason I mention this is because lately I've been finding short hop Nair to be quite a useful OoS tool, especially considering how few viable OoS options Ganon has. It feels weird to use Nair OoS, especially considering that the useful hitbox for using OoS (the one that comes out frame 7) is quite weak, and not where you'd expect to find it on Ganon's body. It's kinda below his bottom leg, and it's weak but not weak enough to get punished, since it hits at a low angle. If you're confused right now, test it out in training... the first hotbox comes out quite a bit before the first "whoosh" sound of the Nair. You may feel a bit insecure using this hitbox since it isn't very strong, but it gets the job done. Learn to use this hitbox (make sure you do SHORT HOP Nair--though full hop Nair is also viable OoS if your opponent is diagonal from you) and you'll have a decent OoS tool on your hands (or in this case, your legs).
Also, Nair is useful for platform pressure (mostly A. when the first hit will knock them off the platform, or B. you're not confident in punishing your opponent's platform landing with Uair, and you need a longer-lasting hitbox). It's also a great followup to thunderstorm at mid-low %'s, as it will often link directly into a DJ Fair.
Runoff Nair also has its uses in edgeguarding, mostly against characters whose recoveries are greatly hindered if they're interrupted by any hitbox.... i.e., spacies (nair often beats phantasm), Diddy (up B OR side B), Ganon/Falcon, Link/TL, and surely a few others.
Finally, what Gheb said. Rising Nair is good for spacing against characters who have short, non- or slightly- disjointed hitboxes, such as Wario, Lucario, Mario, Luigi, etc.