The OSCE meeting, which has been mired in controversy for weeks, began a day before the one-year anniversary of Russia's war against Ukraine.
Austria issued nine visas to Russians, including six who are under EU sanctions, saying it was obligated to grant the visas because the OSCE is based in Vienna.
But Ukraine and Lithuania said they are boycotting the gathering, a parliamentary assembly of the 57-member OSCE.
The Russian delegation is headed by Pyotr Tolstoy, deputy chairman of the Russian parliament's lower house, who is under international sanctions.
It is the first time Tolstoy and another sanctioned parliamentarian have traveled to an EU country since the invasion.
The session was streamed live, with journalists barred from the chamber.
"The Russian war of aggression in Ukraine has endangered security in Europe and Russia is violating all OSCE principles," parliamentary assembly chair Margareta Cederfelt said at the start of the session.
At the beginning of February, Ukraine and 19 other countries sent a letter of protest to the Austrian government, saying Russia would use the meeting to "sow disinformation, fake news and hate speech".
The UK and Poland refused visas to Russians attending OSCE meetings they hosted last year.
But Austria has said it was "obliged under international law to grant entry to the delegates" as host to the OSCE headquarters, adding that the date "is very unfortunate."
The OSCE was founded in 1975 to foster relations between the Western and Eastern blocs and its current members include NATO countries and allies of Moscow.
The organization sends observers to conflicts, as well as elections around the world. It also runs programs such as to combat human trafficking and ensure media freedom.
Since the start of the war, the OSCE has struggled to operate as Russia has blocked all major decisions, which require consensus.