"It's good to get a game like this under our belts," Osterman said. "It's good to have your backs against the wall every now and then. It shows how we keep fighting. We're not just going to let down because another team is competing with us."
Natasha Watley hit an RBI single off Harding to snap a 0-0 tie in the fifth, and Bustos, the most feared hitter in softball, connected for her 10th career Olympic homer in the sixth.
Earlier, China improved to 2-0 with a 7-1 win over Venezuela.
One day after scoring an Olympic-record 11 runs in a tournament-opening rout of Venezuela, the Americans were blanked for four innings before finally pushing a run across against Harding in the fifth.
Lovieanne Jung walked with one out, and one out later, the US' second baseman stole second. Up came Watley against Harding.
Watley battled to a full count before slapping a single to left-center, scoring Jung without a throw.
The Americans made it 3-0 in the sixth when Bustos, the ponytailed powerhouse, hit her second homer in China.
Jessica Mendoza walked leading off and Bustos, who normally pulls her homers deep over the fences in left, dropped one over the right-field wall, giving the US and Osterman a cushion for the seventh.
"She's a beast when it comes to hitting," Osterman said of Bustos. "You can throw her outside and she can beat you. You can throw it inside and she can beat you. You can throw it in the dirt and she can golf it. You can throw it 10 feet over her head and she can tomahawk it. We all kind of laugh because she can do almost anything that she wants."
