The Scot is still waiting for his first win in charge of the City Ground club and he had some harsh words for his new charges.
"We were woeful in the second half. We were sloppy in everything we did and paid the penalty for it," he said.
"We warned the players at half-time that they had to be professional in the second half but we had to be a lot better than that.
"We gave the ball away cheaply, didn't close them down quickly enough and it was all very half-hearted. Their big lad up front (Matt Smith) caused mayhem in the second half and we didn't deal with it. Our centre-halves had to be ugly and rugged to deal with the threat.
"I was pretty pleased with the first-half performance but we looked a little bit frightened after the break. We allowed them to press us and it unnerved us. Why the players allowed that to happen is a mystery to me."
Oldham had to bounce back from the loss of a 13th-minute goal when striker Smith turned an Andy Reid free-kick into his own net.
But Robbie Simpson struck twice within four second-half minutes to turn the game in the visitors' favour. He equalised in the 54th minute with a 25-yard shot that crept inside Lee Camp's left-hand post and four minutes later produced a looping header to convert a superb cross by Dean Furman.
Forest skipper Danny Collins was sent off in the 60th minute for bringing down Smith, and the home side were punished further when leading scorer Jose Baxter whipped in the free-kick to give Oldham a two-goal cushion.
Substitute Billy Sharp pulled one back in added time with a left-footed finish inside the box but it was too late.