One Lord, One Faith, One Church, One Baptism.
Polycarp, despite the "agreements" among professional ecumenistic theologians such as at Balamand (which, AFAIK, no Orthodox Synod of Bishops has officially accepted), the Orthodox Church believes herself--and only herself--to be the One, Holy, Catholic and Apostolic Church, the ONE Body of Christ. "Valid" sacraments then, in Orthodox belief, cannot be performed outside the One True (Orthodox) Church, the ONE Body of Christ. Therefore, when one is accepted into the Orthodox Church by Holy Baptism, it simply means that whatever happened previously--such as "baptism" in another denomination, even if that denomination be the Roman Catholic Church, is not recognized as Sacramental in the *strict* Orthodox view. Some Orthodox Churches are less strict and receive Roman Catholics (and others) who have maintained an outward form of Trinitarian baptism through the Mystery of Holy Chrismation, thereby filling with Grace the empty form of Baptism given outside the (Orthodox) Church--Chrismation completes Baptism sacramentally (Baptism-Chrismation is sometimes called a "di-une" Mystery as they are always administered together in the Orthodox Church). The Church exercises "ekonomia" in this regard--she is the guardian of the sacramental Mysteries. This may be hard to hear, particularly for a Roman Catholic, but that's the way it is, and I would be less than honest to tell you otherwise.