If a bishop is present, he is the one who serves the Divine Liturgy, unless he chooses to delegate someone else to do it. The other services he will usually delegate to a priest. I know one bishop who always chants the Vigil on kliros every weekend with the choir, while the dean of the cathedral is the principal celebrant.
In Orthodox ecclesiology, a priest is only the bishop's representative. He can only operate within the duties delegated specifically to him.
While bishops do have administrative responsibilities, according to what they are charged with at their consecration, it is the *spiritual* oversight of their diocese that they are responsible. Good bishops tend to delegate most administrative responsibilities (while keeping a close watch), and concentrate on prayer, the services, and direct ministry to the people of their dioceses.
In modern (i.e. that of the last 1000 years or so) Orthodox practice, dioceses are fairly large and have many parishes. In our diocese, it is an accomplishment for the bishop to make it to each parish in the diocese once a year. The clergy go to gatherings with the bishop at least twice a year, and laity are also welcome to come to these assemblies.
So, priests do handle the day-to-day operations of a parish, including preaching, in most cases. Historically it has been fairly common for priests not to preach, though. Preaching would be done by those especially delegated by the bishop to do so, and sometimes the bishop would distribute sermons to be read.
I would also point out that preaching is not really appointed to be a part of the Divine Liturgy. Although it is usually done there today here in America, that's not where it belongs, according to the service books. The Divine Liturgy has its own life, and transcends time, and preaching interrupts that. The traditional places for sermons are at Matins. But, because attendance is overwhelmingly best at Liturgy, sermons have moved in recent times into the Divine Liturgy, after the Gospel, or preferably, after the conclusion of the Divine Liturgy.
Yes, homily, sermon, preaching are all the same things. I would also point out that there is another strong tradition in the Orthodox Church that has been lost in the last century or so, and that is the reading of liturgical sermons. It is usually hard for most priests to improve upon the words of the Fathers. I always deeply respect priests who draw heavily from patristic sermons. There are sermons and Scriptural explanations and lives of saints appointed to be read in Church every day of the year. This practice is used too little these days.
And of course, even without preaching or the reading of liturgical sermons, the texts of the services themselves are so chock-full of theology that one can go on overload just listening attentively to the prayers, joining in with one's heart (or lips, if you happen to know the particular hymn being chanted.)
This is the earliest record of Christian liturgy, from early 2 century. Note that the preaching is only done when the time permits. The prayers and the Eucharist are mentioned as central part of the service.
CHAPTER LXV -- ADMINISTRATION OF THE SACRAMENTS.But we, after we have thus washed him who has been convinced and has assented to our teaching, bring him to the place where those who are called brethren are assembled, in order that we may offer hearty prayers in common for ourselves and for the baptized [illuminated] person, and for all others in every place, that we may be counted worthy, now that we have learned the truth, by our works also to be found good citizens and keepers of the commandments, so that we may be saved with an everlasting salvation. Having ended the prayers, we salute one another with a kiss. There is then brought to the president of the brethren bread and a cup of wine mixed with water; and he taking them, gives praise and glory to the Father of the universe, through the name of the Son and of the Holy Ghost, and offers thanks at considerable length for our being counted worthy to receive these things at His hands. And when he has concluded the prayers and thanksgivings, all the people present express their assent by saying Amen. This word Amen answers in the Hebrew language to genoito [so be it]. And when the president has given thanks, and all the people have expressed their assent, those who are called by us deacons give to each of those present to partake of the bread and wine mixed with water over which the thanksgiving was pronounced, and to those who are absent they carry away a portion.
CHAPTER LXVI -- OF THE EUCHARIST.
And this food is called among us Eukaristia [the Eucharist], of which no one is allowed to partake but the man who believes that the things which we teach are true, and who has been washed with the washing that is for the remission of sins, and unto regeneration, and who is so living as Christ has enjoined. For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Saviour, having been made flesh by the Word of God, had both flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh. For the apostles, in the memoirs composed by them, which are called Gospels, have thus delivered unto us what was enjoined upon them; that Jesus took bread, and when He had given thanks, said, "This do ye in remembrance of Me, this is My body;" and that, after the same manner, having taken the cup and given thanks, He said, "This is My blood;" and gave it to them alone. Which the wicked devils have imitated in the mysteries of Mithras, commanding the same thing to be done. For, that bread and a cup of water are placed with certain incantations in the mystic rites of one who is being initiated, you either know or can learn.
CHAPTER LXVII -- WEEKLY WORSHIP OF THE CHRIS- TIANS.
And we afterwards continually remind each other of these things. And the wealthy among us help the needy; and we always keep together; and for all things wherewith we are supplied, we bless the Maker of all through His Son Jesus Christ, and through the Holy Ghost. And on the day called Sunday, all who live in cities or in the country gather together to one place, and the memoirs of the apostles or the writings of the prophets are read, as long as time permits; then, when the reader has ceased, the president verbally instructs, and exhorts to the imitation of these good things. Then we all rise together and pray, and, as we before said, when our prayer is ended, bread and wine and water are brought, and the president in like manner offers prayers and thanksgivings, according to his ability, and the people assent, saying Amen; and there is a distribution to each, and a participation of that over which thanks have been given, and to those who are absent a portion is sent by the deacons. And they who are well to do, and willing, give what each thinks fit; and what is collected is deposited with the president, who succours the orphans and widows and those who, through sickness or any other cause, are in want, and those who are in bonds and the strangers sojourning among us, and in a word takes care of all who are in need. But Sunday is the day on which we all hold our common assembly, because it is the first day on which God, having wrought a change in the darkness and matter, made the world; and Jesus Christ our Saviour on the same day rose from the dead. For He was crucified on the day before that of Saturn (Saturday); and on the day after that of Saturn, which is the day of the Sun, having appeared to His apostles and disciples, He taught them these things, which we have submitted to you also for your consideration.
I also appreciate when my Pastor does our version of this. I like it when he just quotes directly from scripture to make a point, instead of trying to "improve" on it. Thank you very much for all of your comments.