Almost all of the Orthodox opinions have one thing in common - this isn't part of our eastern tradition of theological expression. There are Orthodox fathers (at least one that I know of for sure) that say that Mary was sanctified and filled with grace at least from her early childhood, but don't claim her as being unaffected by the fall or having a nature other than what she received from her parents. The miracle of her conception isn't taught as being (at least in part) free from original sin, but that her mother was barren and past childbearing years and is compared to Abraham and Sarah conceiving Isaac.
Some of the Greek and Eastern fathers (who belonged to an entity called the Catholic Church) did say that Mary was free from sin from conception,but they did not discuss this as an official doctrine of the whole Church or come to a consensus about it.
"Thou alone and thy Mother are in all things fair, there is no flaw in thee and no stain in thy Mother." Ephraem, Nisibene Hymns, 27:8 (A.D. 370).
"As he formed her without my stain of her own, so He proceeded from her contracting no stain." Proclus of Constantinople, Homily 1 (ante A.D. 446).
"A virgin, innocent, spotless, free of all defect, untouched, unsullied, holy in soul and body, like a lily sprouting among thorns." Theodotus of Ancrya, Homily VI:11(ante A.D. 446).
"The angel took not the Virgin from Joseph, but gave her to Christ, to whom she was pledged from Joseph, but gave her to Christ, to whom she was pledged in the womb, when she was made." Peter Chrysologus, Sermon 140 (A.D. 449).
"The very fact that God has elected her proves that none was ever holier than Mary, if any stain had disfigured her soul, if any other virgin had been purer and holier, God would have selected her and rejected Mary." Jacob of Sarug (ante A.D. 521).
"She is born like the cherubim, she who is of a pure, immaculate clay." Theotokos of Livias, Panegyric for the feast of the Assumption, 5:6 (ante A.D. 650).
"Today humanity, in all the radiance of her immaculate nobility, receives its ancient beauty. The shame of sin had darkened the splendour and attraction of human nature; but when the Mother of the Fair One par excellence is born, this nature regains in her person its ancient privileges and is fashioned according to a perfect model truly worthy of God.... The reform of our nature begins today and the aged world, subjected to a wholly divine transformation, receives the first fruits of the second creation." Andrew of Crete, Sermon I, On the Birth of Mary (A.D. 733).
"O most blessed loins of Joachim from which came forth a spotless seed! O glorious womb of Anne in which a most holy offspring grew." John of Damascus, Homily I (ante A.D. 749). http://www.newadvent.org/cathen/07674d.htm
The absolute purity of Mary
Patristic writings on Mary's purity abound.
The Fathers call Mary the tabernacle exempt from defilement and corruption (Hippolytus, "Ontt. in illud, Dominus pascit me");
Origen calls her worthy of God, immaculate of the immaculate, most complete sanctity, perfect justice, neither deceived by the persuasion of the serpent, nor infected with his poisonous breathings ("Hom. i in diversa");
Ambrose says she is incorrupt, a virgin immune through grace from every stain of sin ("Sermo xxii in Ps. cxviii);
Maximus of Turin calls her a dwelling fit for Christ, not because of her habit of body, but because of original grace ("Nom. viii de Natali Domini");
Theodotus of Ancyra terms her a virgin innocent, without spot, void of culpability, holy in body and in soul, a lily springing among thorns, untaught the ills of Eve, nor was there any communion in her of light with darkness, and, when not yet born, she was consecrated to God ("Orat. in S. Dei Genitr.").
In refuting Pelagius St. Augustine declares that all the just have truly known of sin "except the Holy Virgin Mary, of whom, for the honour of the Lord, I will have no question whatever where sin is concerned" (On Nature and Grace 36).
Mary was pledged to Christ (Peter Chrysologus, "Sermo cxl de Annunt. B.M.V.");
it is evident and notorious that she was pure from eternity, exempt from every defect (Typicon S. Sabae);
she was formed without any stain (St. Proclus, "Laudatio in S. Dei Gen. ort.", I, 3);
she was created in a condition more sublime and glorious than all other natures (Theodorus of Jerusalem in Mansi, XII, 1140);
when the Virgin Mother of God was to be born of Anne, nature did not dare to anticipate the germ of grace, but remained devoid of fruit (John Damascene, "Hom. i in B. V. Nativ.", ii).
The Syrian Fathers never tire of extolling the sinlessness of Mary. St. Ephraem considers no terms of eulogy too high to describe the excellence of Mary's grace and sanctity: "Most holy Lady, Mother of God, alone most pure in soul and body, alone exceeding all perfection of purity ...., alone made in thy entirety the home of all the graces of the Most Holy Spirit, and hence exceeding beyond all compare even the angelic virtues in purity and sanctity of soul and body . . . . my Lady most holy, all-pure, all-immaculate, all-stainless, all-undefiled, all-incorrupt, all-inviolate spotless robe of Him Who clothes Himself with light as with a garment . . . flower unfading, purple woven by God, alone most immaculate" ("Precationes ad Deiparam" in Opp. Graec. Lat., III, 524-37).
To St. Ephraem she was as innocent as Eve before her fall, a virgin most estranged from every stain of sin, more holy than the Seraphim, the sealed fountain of the Holy Ghost, the pure seed of God, ever in body and in mind intact and immaculate ("Carmina Nisibena").
Jacob of Sarug says that "the very fact that God has elected her proves that none was ever holier than Mary; if any stain had disfigured her soul, if any other virgin had been purer and holier, God would have selected her and rejected Mary". It seems, however, that Jacob of Sarug, if he had any clear idea of the doctrine of sin, held that Mary was perfectly pure from original sin ("the sentence against Adam and Eve") at the Annunciation.
St. John Damascene (Or. i Nativ. Deip., n. 2) esteems the supernatural influence of God at the generation of Mary to be so comprehensive that he extends it also to her parents. He says of them that, during the generation, they were filled and purified by the Holy Ghost, and freed from sexual concupiscence. Consequently according to the Damascene, even the human element of her origin, the material of which she was formed, was pure and holy. This opinion of an immaculate active generation and the sanctity of the "conceptio carnis" was taken up by some Western authors; it was put forward by Petrus Comestor in his treatise against St. Bernard and by others. Some writers even taught that Mary was born of a virgin and that she was conceived in a miraculous manner when Joachim and Anne met at the golden gate of the temple (Trombelli, "Mari SS. Vita", Sect. V, ii, 8; Summa aurea, II, 948. Cf. also the "Revelations" of Catherine Emmerich which contain the entire apocryphal legend of the miraculous conception of Mary.
From this summary it appears that the belief in Mary's immunity from sin in her conception was prevalent amongst the Fathers, especially those of the Greek Church. The rhetorical character, however, of many of these and similar passages prevents us from laying too much stress on them, and interpreting them in a strictly literal sense. The Greek Fathers never formally or explicitly discussed the question of the Immaculate Conception.