The international security organisation's chief said the move would send a message to Russia that there are "costs and consequences" for its behaviour.
Twenty-six countries have expelled Russian envoys in the past two days, in solidarity with the UK.
They all believe Russia was behind the poisoning of two people in Salisbury.
Russian ex-spy Sergei Skripal and his daughter, Yulia, were poisoned on 4 March in the southern English city, and investigators say a military-grade nerve agent was used.
Russian has denied involvement.