Al-Shami said the rebels withdrew from the base after they captured some ammunition. He said they feared counterstrikes by regime aircraft.
An amateur video posted online showed rebels walking next to two destroyed helicopters. At least three other helicopters appeared undamaged. Black smoke billowed in the distance.
Another video showed several radar posts on hills inside the large compound. Parked military trucks stood inside as rebels roamed freely.
The Observatory also reported violence in other parts of Syria, including the country's largest city of Aleppo in the north and the capital itself.
It said rebels on Sunday captured a training base for the Popular Front for the Liberation of Palestine-General Command near the Damascus suburb of Douma. The PFLP-GC is one of the Palestinian factions most loyal to Assad.
The PFLP-GC said in a statement late Saturday that the base was under attack. It said that thousands of activists and fighters who fought against Israel were trained at the base over the past 30 years.
Also Sunday, the Observatory said a bomb targeted a bus in the southern village of Othman, killing at least five people and wounding dozens. It said rebels and troops clashed in the southern region of Quneitra on the edge of the Israeli-occupied Golan Heights.
The Local Coordination Committees, another activist group, said residents found 12 bodies in the Damascus suburb of Daraya, scene of heavy clashes between rebels and government troops over the past few days.
State TV said troops clashed with al-Qaida militants in Daraya, killing some of them and confiscating a mortar that they were using in their attacks.
The station said that troops killed an al-Qaida affiliated Palestinian militant known as Abu Suhaib in the Damascus suburb of Hajira. It said his group was behind several bombings in Syria that killed and wounded dozens of people.
Assad's regime blames the revolt on a foreign conspiracy. It accuses Saudi Arabia and Qatar, along with the United States, other Western countries and Turkey, of funding, training and arming the rebels, whom it calls terrorists.