I am a Syriac Orthodox from Malankara (India);
As far as I know the teaching of the Church is that we are not guilty of the sins of Adam but still we suffer the consequence of his sin, which is a corruption of the image of God in which we were originally created. I dont know if that is what the RC or EO teaches.
Regarding, mortal sin; I am guessing you are referring to what is unforgivable sin. The only unforgivable sin is to deny the Holy Spirit. In other words, to not to accept the mercy and forgiveness that is freely offered by God. In other words there is no sin that our Lord will not forgive, if we repent and trust in His mercy to forgive. The only thing that He wont forgive is that if we dont trust in His mercy and dont ask for forgivness; which is to deny the Holy Spirit. On this also I dont know what the RC or EO teaches.
I have to also say that I am not theologically trained and not qualified to comment on theological positions of the church. I just tried to summarize my recollection of what I have heard in church and church school.
I will wait for more authoritative comments from more learned OO on this forum like Fr. Peter
Mathew G M