The U23 champions have their task cut out to win over Bahrain in their open group now to reach semis.
UAE's false start to Gulf Cup defence
UAE 2 // Oman 3
UAE (Al Fardan 40P, Al Gharib 83)
Oman (Al Sadi 6, Al Hadari 44, Al Shamli 47)
Man of the match: Mansoor Salim (Oman)
The UAE's title defence of the Under 23 Gulf Cup was dealt a severe blow when Oman stunned them 3-2 at the Aspire Academy in Doha, Qatar.
Oman scored twice before the break and again in the second minute of the second half to bounce back from their 1-0 defeat to Bahrain in the opening night of the championship, leaving Group B wide open.
The UAE will now have to win by more than a one-goal margin against Bahrain on Tuesday to secure one of the two semi-final berths in the three-team group.
Oman took the lead after a flowing move from a counter attack. Humaid Al Sadi, unmarked in the centre, completed the move by shooting past Ahmed Mahmoud, the UAE goalkeeper, for a sixth minute lead.
Habeeb Al Fardan struck the equaliser from a spot kick when Rashid Essa was brought down by an Omani defender five minutes from half time.
Hussain Al Hadari regained the lead a minute before the break as the UAE defence were caught napping again.
Nasser Al Shamli consolidated Oman's lead when he headed a corner two minutes after the break and the UAE closed the gap through Mohammed Al Gharib seven minutes from time, but that was as close they got as Oman held on to their one-goal lead.
The UAE could have cancelled Oman's early lead had Ali Mabkhout been on target from inside the box. The Al Jazira forward's effort sailed over the crossbar. Mohammed Abdulrahman then shot straight at the goalkeeper Ahmed Younus, who also did well to cling on to a stinging shot from Al Fardan.
While the UAE forwards failed to hit the target, the defence leaked out goals, twice from the middle and one off a corner, leaving Mahdi Ali, the coach, with some serious work ahead of their clash with Bahrain.