The 30-year-old Japanese southpaw will face Mexico's Raymundo Beltran on May 1 for the vacant World Boxing Organization lightweight crown, launching an eight-night US television boxing series from promoters Top Rank.
Ao, 27-3 with one drawn and 16 knockouts, won the World Boxing Council featherweight crown in 2009 and held the WBC super featherweight title from 2010-2012.
Beltran, 33, is 29-7 with one drawn and 17 knockouts. He failed twice before when fighting for the WBO lightweight crown, drawing with Scotsman Ricky Burns in 2013 -- a highly controversial fight that even Burns's own promoter Eddie Hearn admitted Beltran deserved to win -- and losing last November to unbeaten American Terence Crawford.
The bout will serve as an appetizer for the May 2 showdown between unbeaten Mayweather, 47-0 with 26 knockouts, and Filipino icon Pacquiao, 57-5 with two drawn and 38 knockouts, at the MGM Grand for the WBC, WBO and World Boxing Association welterweight crowns.