FSA appoints new chief
The appointment comes after former head General Selim Idriss had initially refused to step down.
Bashir, who defected from the government army to join rebels fighting in southern Syria, was originally appointed to the position in February after Idriss was dismissed from his position.
The FSA's Higher Military Council removed Idriss, citing the "difficulties faced by the Syrian revolution" in its battle with the regime of President Bashar al-Assad.
Idriss and other senior commanders rejected the dismissal and said the move was a ‘coup.”
But earlier this week, opposition chief Ahmad Jarba had met with several top military leaders, including Idriss and Mustafa.
During the meeting it was agreed both Idriss and Mustafa would quit their positions and Idriss would be named military advisor to Jarba.
Considered the "moderate" rebel group, the Western-backed FSA was once the country's strongest armed opposition force but is now increasingly marginalized by Islamists.