Actually, I think the use of the terminology, "offend God" is a bow to Protestant misunderstandings of some supreme notion of being "separated" from God.
The Orthodox, AFAIK, do not distinguish between mortal and venial sins; and the distinction is even a dangerous one for modern times. A better distinction would be between "grave" sins and "venial" sins; a "grave" sin being one with the severity to be mortal, but not necessarily the requisite deliberateness or knowledge to be mortal.
The Catholic distinction is based on the notion that certain sins, if deliberate, chosen of free will, and with full understanding of their severity, signify a refusal of grace.
Dangus points up another distinction between Orthodox and Western ideas about sin. As the priest says in confession, our sins are forgiven whether known or unknown. Missing the mark does not require a conscious effort on our part.This is consistent with Orthodox theology on how we are judged at the Final Judgment, not whether we have fasted, done good deeds or failed to fast or committed evil deeds but rather we are scrutinized to determine of we have any similitude to Christ. God loves all of us and his grace falls on the good and the evil alike. But in the end, it is that divine love which scourges those how have rejected God and becomes a torment to them, just as that same love creates Paradise for those who have approached or attained theosis.