Is Oskar your only animal?
Lotte did that most of last summer (her first summer), a cough like she's clearing her throat sort of after running/being outside (in addition to reverse sneezing, but the reverse sneezes stopped long ago). Clean bill of health from her vet, no coughing all winter...
Some process of elimination, because it only happens at home around the cats, and I'm pretty sure, in her case, that it's just cat hair she's picking up.
Could also be pollen/grass particles/sand getting in his throat when he's outside, especially if he's running around with his mouth open. Or just a dry throat.
Anyway, even though he was just seen, you could always have the vet check him out again if it'll make you feel better. As long as he got the
though, I wouldn't worry too much about it.