Oswald was there and was totally into all of the muggle pop music, and probably drank quite a bit and was very sociable - in short, quite the opposite of what he was like as a prefect at school. He would have tried to talk enthusiastically to Rachel/Ritchie/jack/Geoffrey et al about muggle music if he got the chance, and probably brought along something as a hostess gift, like a fancy cake or cupcakes or you know, whatevs. He was pretty excited to have been invited!