Oates came out of the dugout in the fifth inning and asked the umpiring crew to check Downs' glove for evidence, but none was found.
At the end of the inning, as Downs returned to the Oakland dugout, a television camera appeared to capture the right-hander making a motion that resembled placing an object inside his shirt.
"He was scuffing the ball with sandpaper or something," said Oates. "Everybody all over the country saw him put it down his shirt on TV. But the umpires are helpless. There's nothing they can do about it. They can't touch him."
Plate umpire Joe Brinkman, the crew chief, said his check of Downs yielded no foreign substance.
"That's all you can do is go out and check him," said Brinkman. "All you can do is look.
Downs expressed surprise at the charge.
"I have no idea," said Downs. "I throw a four-seamer [fastball] that goes away from right-handers and I throw a two-seamer that sinks. If they want to think it, that's fine."
Catcher Chris Hoiles suffered a broken wrist last June by a pitch thrown by then-New York Yankees pitcher Tim Leary that the Orioles contended was doctored, though the umpiring crew that night could find no evidence of markings.
Coincidentally, Hoiles was batting last night when Oates went to Brinkman to accuse Downs.
Oates claimed that Downs, who got the win last night, working 2 2/3 innings, yielding one hit and no runs, had done the same thing in Oakland last weekend.
"He was doing it in Oakland. I had six baseballs with exactly the same mark on them," said Oates. "But after we took the sandpaper off, we still didn't hit him. That's not the reason we lost the ballgame."