The visitors looked set to pick up a 9-5 victory over their hosts, who had earlier been reduced to 14 men after Steff Evans was sent off for an aerial challenge on rival wing Ben John, but Josh Macleod clinched the points at the death.
Ospreys had failed to make their numerical advantage count but Tandy felt they did not deserve to lose the game.
"It's pretty disappointing losing that game," Tandy said after the 12-9 loss. "I thought tactically in the second half we were pretty good. We were in control, we felt we had the scrum and we could see the game out from there.
"But in fairness the Scarlets came back and they kept playing and got the try.
"I thought some of our scrum stuff was pretty decent until the last one. On reflection we just nudged the game but we didn't get the win. The conditions were awful."