Raed Saleh and fellow White Helmet member Khaled Khatib had been set to attend Sunday's ceremony in Hollywood, where "The White Helmets" is shortlisted for best short documentary.
"After 3days at airport, not allowed to travel to #oscars2017 - had US visa - but passport not accepted. Sad, but important work to do here," Khatib tweeted Saturday from Istanbul.
US Customs and Border Protection spokesman Michael Friel said that "individuals need valid documents to travel to the United States," declining to comment further.
Earlier, Saleh told AFP he wasn't going because of a heavy workload "due to intensified regime strikes on the provinces of Damascus, Daraa and Homs."
"There are many things that have to be done on the ground, such as managing operations and providing emergency vehicles," he added.
Khatib, who shot much of the footage, had obtained a US visa but cited the same reasons earlier for not traveling.
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"I won't travel to OSCAR due to intensity of work, our priority is helping our people," he tweeted in English.
"I was going to leave on Tuesday, but there was too much work because of the bombing," Khatib told AFP by telephone.
"I'm also working on producing another film about the White Helmets that has to be ready in two weeks. I will not go."
The two rescuers had feared they could be barred from attending the ceremony because of US President Donald Trump's late January executive order imposing a 90-day entry ban for citizens of seven Muslim majority countries, including Syria.
The ban has since been lifted by a US federal court, and on February 18 the pair received their visas.
More than 310,000 people have died since Syria's conflict erupted in March 2011, and more than half of the country's citizens have been forced to flee their homes.
Since the White Helmets group was created in 2013, it has hired more than 3,000 volunteers and claims to have saved more than 78,000 lives.
It takes its name from the protective headgear worn by its members.