He was joined on the scoresheet by fellow Wales international Eli Walker, Jonathan Spratt and Jeff Hassler, as Kayle van Zyl's try-double for Zebre proved in vain.
It means Tandy's men have now strung together three GUINNESS PRO12 victories in a row and are now just even points behind the top four.
And Tandy was left in no doubt who the star of the show was.
"Justin is a class act. When he plays like that he can be pretty much unplayable," he said.
"He leads from the front and the way in which he plays the game - he doesn't say a lot but when he does speak everyone listens."
Attention now turns to European action over the next fortnight and first up for Ospreys is a visit from Bordeaux-Begles at the Liberty Stadium on Saturday night.
"It's a six-day turnaround but we were able to rest some of the players who played in the World Cup so we should be in good shape for that," added Tandy.
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