Note: I am almost certain this topic has been heavily discussed, but I searched and can't find anything. Feel free to point me to a previous thread dealing with this. I am writing from the perspective of an Antiochian Orthodox.
Met. Kallistos Ware says the following in his book The Orthodox Church:
"To the question how one can know whether a council is ecumenical, Khomiakov and his school gave an answer which at first site appears clear and straightforward: a council cannot be considered ecumenical unless its decrees are accepted by the whole Church. Florence, Hieria, and the rest, while ecumenical in outward appearance, are not truly so, precisely because they failed to secure this acceptance by the Church at large."
If a council is only deemed ecumenical once the whole Church has accepted it, how does this work with Chalcedon? The OO, who were at the time still in communion with the EO, did not accepted this council, yet it is considered ecumenical by the EO, myself included.
The only way I could think of is if the OO decided to leave the Church over this, rather than merely not accepting it.