The match turned out to be a bloody one, with Omega busting open Fenix's nose at one juncture. Omega won with the One Winged Angel.
In the post-match, Omega blew off a handshake attempt from Fenix, instead holding the belt aloft.
Omega issued the challenge for the match at AAA's TripleMania XXVII event in early August. On that show, Fenix teamed with Pentagon Jr and Laredo Kid to defeat Omega and The Young Bucks.
There exists a working relationship between All Elite Wrestling and AAA, which was announced back in February.
This marks Omega's first time holding a championship since losing the IWGP Heavyweight title to Hiroshi Tanahashi at Wrestle Kingdom 13 on January 4.
Fenix's 420-day reign, which began at last year's TripleMania, was the second longest of any AAA Mega champion, and one of only two reigns with that belt to surpass one full year.