The competition newcomers went a man down when German defender Jerome Polenz was sent off in the 13th minute for bringing down Heart forward David Williams from behind in a goal-scoring position. Ono's 73rd-minute penalty for his fifth goal of the season secured the backs-to-the-wall win, after Croatia's Josip Tadic earlier had a spot kick saved following Polenz's sending off.
The win was Western Sydney's third straight and they moved ahead of Adelaide United into third on goal difference. Heart's failure to capitalise on the undermanned Wanderers left them in danger of dropping out of the top six over the remainder of the weekend.
Earlier, Melbourne Victory enjoyed a 3-1 win over Sydney FC, who finished with nine men after Brazilian defenders Tiago Calvano and Fabio were red-carded. Second-half efforts from Archie Thompson and Marco Rojas, after a 24th-minute deflected own goal from Sydney defender Seb Ryall, capped a dominant performance from the Victory.
Rojas would have impressed scouts from English Premier League club Liverpool and Italian giants Juventus in attendance with a magnificent display and capped off by a beautifully-crafted goal in the 72nd minute. Sydney's Italian superstar Alessandro Del Piero was kept in check by Victory's midfield.