Can a Catholic marry a Orthodox? What are the rules about it?
Yes, according to the Catholic Church, a Catholic can marry an Orthodox. (Whether a particular Orthodox church will allow one of its members to marry a Catholic is another question and can only be answered by an authority of that particular Orthodox communion.) For a Catholic to marry an Orthodox, he must receive a dispensation from his local bishop to marry a non-Catholic. If they plan to marry in a non-Catholic ritual – such as in the church of the Orthodox Christian – the Catholic must also receive a dispensation from his bishop to marry in a non-Catholic ritual.
That said, interfaith marriages are extremely difficult if either party is attached to his or her faith. (If both are, it becomes even more difficult.) And, keep in mind that attachments to faith can change once children come along and parents start contemplating passing on religious heritage to the children. All in all, it is best to marry someone within one’s own faith tradition in order to best help each other and children achieve salvation (which is, after all, the primary purpose of marriage).