Pistorius has mostly stayed out of the public eye since the Valentine's Day shooting of his girlfriend, Reeva Steenkamp, and his subsequent arrest and release on bail.
But South African press reported Pistorius "knocking back" shots at a popular spot, Kitchen Bar, looking "relaxed and happy."
One woman told the Sunday Times he "didn't look like someone [who had] lost the love of his life."
"I can't believe he showed his face in public, carrying on as if nothing's happened," she said. "He was greeting a couple and then gave the woman a little pat on her backside. It was just so inappropriate."
Pistorius's spokeswoman shot down implications that his behavior was inappropriate.
He "strenuously and vigorously denies any inappropriate behavior towards people or women present, or 'partying it up' with shooters," spokeswoman Anneliese Burgess told the Sunday Times.
"Oscar did go out last Saturday with friends," she said. "He has been out of the house with family, but this was the first time he went out to a restaurant with people other than close family."
"It is regrettable that what was supposed to be a low-key evening reconnecting with friends is now being blown out of proportion for sensational reasons."
Pistorius spent about 40 minutes at Kitchen Bar, and left when "public interest became overwhelming." He then spent another half hour at a bar across the street.