O'Connell, accompanied by his solicitor Donal Spring and Munster team manager Shaun Payne, contested the red card.
The ERC disciplinary officer, Roger O'Connor, presented the case against the player.
After hearing submissions from both parties and considering the evidence, the independent judicial officer, HHJ Jeff Blackett from England, upheld the red card and determined that the act had been deliberate, but that there had been no intention to injure Thomas.
The independent judicial officer found that the offence was in the mid range (entry point: four weeks) of the level of seriousness for an offence of this type.
Having taken into account any mitigating factors, such as O'Connell's good disciplinary record, O'Connor reduced the suspension from the entry point by one week which resulted in the final suspension of four weeks.
O'Connell, who is just back from a long-term groin injury, will miss the return match against the Ospreys this Saturday and three Magners League games against Connacht, Ulster and Glasgow Warriors. He will be free to play from January 10.
After the hearing, Shaun Payne said that the province would wait until they received the full judgement before deciding whether or not to launch an appeal.