The Leicester City manager branded French official Remi Harrel as "desperately poor" after he had sent off Garry Parker for a second bookable offence - taking a free kick too quickly.
Atletico won that match 2-0 to take the tie 4-1 on aggregate.
Uefa's disciplinary committee met yesterday and a spokesman said: "Mr O'Neill's explanation has been accepted and there will be no sanctions taken against him."
Wolverhampton Wanderers have signed the Derby County winger Paul Simpson on loan and the Molineux manager, Mark McGhee, is hoping to secure a permanent deal.
Simpson, who is valued at around pounds 75,000, goes straight into the squad for tomorrow's West Midlands derby with Birmingham City at St Andrew's.
The former Manchester City and Oxford United player has been out of the first-team picture at Pride Park this season, but has impressed McGhee in the reserves.
"They don't want a lot of money for him and what I said to the boy is that I don't want him here if all he is going to do is clutter up the place by not being in the team," the Wolves manager said. "But, if he comes and proves he can make a contribution this season, then he can stay and he is up for the challenge. He wants to play first-team football."
The Welsh international defender Adrian Williams has returned to Molineux after being pulled out of the World Cup qualifier with Belgium because of an Achilles injury.
The Bolton manager Colin Todd has denied reports that the midfielder Jamie Pollock has asked to go on the transfer list. "The situation is that Jamie has been doing a job for the team and has been asked to play out of position," Todd said.
"The player is disappointed and unhappy, but we've talked it through and I've agreed that, if an offer comes along that is right for the club and the player, then we will listen to it."
The Manchester City manager Frank Clarke is facing a selection crisis after Uwe Rosler was ruled out of action for six weeks. Clarke may be forced to buy a replacement for the German forward, who damaged ankle ligaments in a friendly with an Italian select XI this week.
The transfer-listed Rosler will not be available again until late November, which will also put an end to his chance of a move to Everton.
Rosler's lay-off leaves City short of bodies. Paul Dickov went to hospital for a scan on his knee yesterday after picking up an injury in the same game and could need a minor operation. That would leave City with just Lee Bradbury in attack as they try to escape the bottom half of the First Division.
The West Bromwich Albion striker Paul Peschisolido has rejected an improved contract with the club. He had said earlier in the week that he was "very close" to signing a two-year extension to his current deal, which has 18 months to run but talks have broken down.
Peschisolido, who dropped his agent Eric Hall from the talks after he was involved in a row with Albion's chief executive, John Wile, said: "I met with chairman Tony Hale but we could not reach agreement."Reuse content