All marriages are considered valid if undertaken with the proper form and with the appropriate capacity.
What this means is, essentially, if the marriage is a first marriage, it is valid for the people given the state of life they were in when they were married, and it was undertaken freely, the Church generally assumes it to be a valid marriage.
This is in contrast to something like a Catholic being married outside the Church without a dispensation, or marrying someone who is already married (divorces do not undo the marriage). In those cases the marriage is inherently invalid.
In the case of Orthodox marriages, as long as the two people are on their first marriage each, or their previous marriage has been annulled, and as long as they follow the forms required of them as Orthodox Christians, then the Church would accept their marriage as valid and binding.