In professional Japanese sumo the wrestlers - sumotori - are divided into ranks, depending on their skills. In the top division (the makuuchi) are the maegashira, and in the top part of the makuuchi, called sanyaku we find the komusubi, the sekiwake, and the ozeki. On top of that sit the yokozuna, by many definitions not really part of sanyaku.
An "ozewake" is an upwardly mobile sekiwake. That is to say: a sumotori ranked as sekiwake who is, again and again, on the verge of being promoted to ozeki - but who just seems to lack that last certain something; whose score is just a little too low, too often, or whose sumo is just not consistent enough to cut it.
"Ozewake" is not a rank, and the expression probably shouldn't be used too freely lest you invoke the wrath of a sumo wrestler who has just, once more, missed being promoted to the coveted rank of ozeki.
My source for this one is, besides hours upon hours of watching sumo: Sumoforum's wonderful Glossary.