Orthodox do distinguish between mortal and venial sins

Concerning Mortal Sins, Pardonable Sins, and Sins of Omission
Part I, Chapter 3 from the Exomologetarion (A Manual of Confession)
by St. Nikodemos the Hagiorite

"Mortal sins are those voluntary sins which either corrupt the love for God alone, or the love for neighbor and for God, and which render again the one committing them an enemy of God and liable to the eternal death of hell. "

"Pardonable sins are those voluntary sins which do not corrupt the love for God or the love for neighbor, nor do they render the person an enemy of God and liable to eternal death, to which transgressions even the Saints are susceptible, according to the words of the Brother of God."