The 20-year-old had been training with the squad during their pre-season camp in Spain while awaiting the permit, which has now been granted, allowing him to join the first team at Brisbane Road.
Al Sadi, full name Tariq Khalifa Salim Al-Sadi, joined Orient in 2024 as an 18-year-old and has spent the past two seasons training with the club's youth squads, having previously had spells with Birmingham City, Aston Villa and Wolves as a youngster after moving to England from Muscat, where he first took up football at the age of three before also drawing interest from Emirati side Al-Wasl and going on trial with Fulham.
He also had a brief stint with Spanish side Sant Andreu before returning to east London, and marked his senior Oman debut in May 2025 by scoring a penalty in a 4-1 friendly win over Niger.
"I'm really happy and proud to have signed for the club, I've been here for some time and it feels good to have a deal with Orient," Al Sadi told the club website.
Director of football Scott Mitchell added: "Tariq has worked hard throughout pre-season. It's important for him to continue to develop as a player as we look to secure him a loan move for the new season."
Orient finished 20th in League One under manager Richie Wellens last season, narrowly avoiding relegation, and will be looking to build strength in depth across the squad as they prepare for the new campaign, with Mitchell's comments suggesting a temporary move away from Brisbane Road may follow before Al Sadi is expected to feature regularly for the first team at Orient.