The Britain-based monitor said the deaths were in the Duwaylaa neighbourhood of Damascus, adding that most of those killed appeared to be civilians.
At least 20 people were also injured in the Friday shelling, the monitor said.
Rebels in strongholds on the outskirts of the capital regularly fire rockets into Damascus, often killing scores of people, many of them civilians.
The regime also regularly carries out air strikes on rebel-held areas around Damascus, particularly the Eastern Ghouta region, where aerial assaults in August alone killed 377 people, according to Doctors Without Borders.
Rights groups have criticised both rebels and the regime for indiscriminate fire that largely affects civilians.
The shelling on Damascus came as an Islamist rebel group that often fires on the capital began an assault on Syria’s largest prison.
Jaish al-Islam began the attack on Adra prison near Damascus on Friday, seizing two buildings, according to the Observatory.
Syria’s state news agency SANA made no mention of shelling or deaths in Duwaylaa, though it reported five people had been killed in rocket fire on several other Damascus neighbourhoods on Friday.
More than 240,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011.