Jabo Ibehre almost gave the visitors a perfect start as his early shot drifted just wide of the Robins goal with barely two minutes on the clock.
Goalmouth action was in short supply until the midway point in the first half as James Dayton and Genseric Kusunga went close to giving the visitors a half-time lead, while Agard was inches away from netting for Steve Cotterill's men.
However, the Robins finally hit the front when Agard found space in the area from a free kick to prod the ball past a helpless Paul Rachubka.
The former Rotherham United frontman almost added a second on the hour mark, but saw his close-range effort drift narrowly wide.
Oldham then squandered an opportunity to draw level after Dominic Poleon's shot was stopped by Fielding.
Lee Johnson's men desperately sought a late equaliser and almost got it on 80 minutes when Carl Winchester's strike from the edge of the area flew inches over the crossbar, but City held on to extend their lead at the top of the table to five points from Preston North End.