In 1963, he won the semifinal of the Soviet championship ahead of Boris Spassky, Alexei Suetin, Igor Bondarevsky and Yuri Averbakh, but finished last of 20 in the final.

In 1965, he came third in the semifinal, and achieved his best result in a Soviet championship final, finishing in eighth place behind Leonid Stein, but ahead of David Bronstein and Korchnoi.

In 1968, Osnos finished =1st in the Leningrad City Chess Championship, but lost the playoff against Valery Bykov and Alexander Cherepkov.

[4] Between 1968 and 1974 he was one of Viktor Korchnoi's seconds,[5] and in 1974 he was awarded the title of Honoured Trainer of the Russian SFSR.

Osnos assisted Korchnoi during his narrowly lost World Championship Candidate's final match against Anatoly Karpov in Moscow 1974.