Ota, other wise known as Charles Bellamy, had defended the the title an impressive 8 times following his March 2010 victory over Akio Shibata to win the belt. Amongst those defenses Ota defeated Tadashi Yuba, twice, Akio Shibata and Koji Numata.
Currently ranked #4 by the WBO at Light Middleweight, #9 at Light Middleweight by the WBC and #11 by the WBA at Welterweight Ota does have some doors open for his next step. Of course he is still a few steps from a world title fight though giving up the OPBF title does suggest that he's already in talks for an eliminator of some sort which would be the next major step, unless he has been offered a voluntary title fight.
Of the options out there the only voluntary title fight that seems open is for the WBA Welterweight title, or rather the interim version held by the highly touted Keith Thurman (22-0-0-1, 20). This would seem a plausible option for Ota who could go back to New York, where he was born, and the bout could act as a co-feature with another big name bout at the Barclay's Centre, one of the emerging US venues for boxing.
As for the other two titles however, it seems they are both booked up, for now. The WBO Light Middleweight title will see Demetrius Andrade defending against Brian Rose whilst the WBC Light Middleweight title should, in theory, see Floyd Mayweather defending against Sergey Rabchenko, as unlikely as that bout sounds.
Of course the WBA Welterweight regular title is likely to be booked up by Marcos Maidana's rematch with Adrien Broner. Interestingly that would make a logical co-main feature to a Thurman-Ota bout for the interim title with the winners to meet, though logical doesn't often apply to boxing.