Omar is the first character who was killed in both timelines.
Omar was accidentally killed by his own friends in both timelines. In the original timeline, Keamy killed him unintentionally by kicking the grenade to Omar. In the flash-sideways timeline, he was shot by one of Keamy's henchmen when the henchman was aiming for Sayid.
Anthony Azizi was credited as Omar/Arabic man in the press release for "Tramonto".