Indonesia took the lead in the 13th minute when Luke Vickery broke down the right flank and sent a pass into the box for Oratmangoen to tap in.
The hosts dominated possession in the opening stages and created several chances, but Singapore gradually tightened their defense and limited Indonesia's attacks. Indonesia had 55% possession and six shots in the opening 20 minutes, according to Sofascore, while Singapore failed to register a shot on target.
Singapore's best efforts came through counterattacks, but their players struggled to connect with the forwards. They did not manage a shot on target in the first half.
Indonesia continued to press after the break, while Singapore looked for opportunities on the counterattack. Song Ui-Young's long-range effort in the 58th minute was comfortably saved by goalkeeper Maarten Paes, Singapore's first shot on target.
The game changed in the 70th minute when Singapore defender Safuwan Baharudin was sent off after a VAR review for fouling Mitchell Baker as the last defender in a promising scoring situation.
One minute later, Indonesia was awarded a penalty after Thom Hayes's free kick struck a Singapore player's hand inside the box.
Romeny stepped up and converted from the spot in the 73rd minute, sending goalkeeper Izwan Mahbud the wrong way to make it 2-0.
Singapore, reduced to 10 men, struggled to create chances in the closing stages. Indonesia controlled the game until the final whistle, with six minutes of stoppage time added.
Indonesia coach John Herdman had named a 23-man squad for the tournament, including Europe-based players such as Paes, Kevin Diks, Calvin Verdonk, Elkan Baggott, Justin Hubner and Romeny.
Singapore are coached by 36-year-old Gavin Lee, with midfielder Shah Shahiran, captain Hariss Harun and forward Ilhan Fandi among their key players.