From St. George's site--
What is the meaning of the Oil in the parable of the ten virgins?
On Holy Tuesday the Church calls to remembrance the parable of the Ten Virgins (Matthew 25:1-3); the other the parable of the Talents (Matthew 25:14-30) pointing to the inevitability of the Parousia and the need for proper preparation and vigilance.
We learn at least two basic things. First, Judgment Day will be like the situation in which the bridesmaids (or virgins) of the parable found themselves: some had sufficient oil and were prepared which others did not have enough and their lamps went out before the bridegroom came. The time one prepares for our union with God is now and not at some undefined point in the future.
Those who did not properly prepare went back to get more oil but when they returned the door to the banquet was closed to them. The tragedy of the closed door is this situation is of our own making, not God's. Those who were prepared entered without any problem and joined in the feast. The exclusion from the marriage feast, the kingdom, is of our own making. Second, we are reminded that watchfulness signifies inner tranquility and joy coupled with attentiveness and vigilance. We must have the personal resolve to find and do the will of God, embrace all His commandments and guard the heart and mind from all evil thoughts and actions.
What is the meaning of the Oil in this parable? Some say it a symbol of good works but it's more than this.
Saint Seraphim of Sarov says,
These virgins did good, and out of their spiritual foolishness supposed that doing good was exactly the point of Christianity. They did good works and by this obeyed God, but they did not care in the least beforehand whether they had received or reached the grace of God's Spirit…
It is more than good works that is expected of us. Its the grace of the Holy Spirit that is essential––this is the Oil. He continues, This very gaining of the Holy Spirit is that oil which the foolish virgins lacked. They were called foolish because they forgot about the necessary fruit of virtue, the grace of the Holy Spirit, without which no one is saved and no one can be saved, for: ‘it is by the Holy Spirit that any soul is vitalized and exalted in chastity, and any soul is lit by the Trinitarian unity in holy mysteries’...
What is this market place where the foolish virgins went to get their oil? What was the closed door?
The foolish virgins, seeing that their lamps were going out, went to the marketplace to buy oil but would not come back in time, for the doors were already shut. The marketplace is our life; the door of the house of marriage (that was shut and did not lead to the Bridegroom) is our human death; wise and foolish virgins are Christian souls; the oil is not works but the grace of the All-Holy Spirit of God which is received through these works, and which converts things perishable into things imperishable, transforms spiritual death into spiritual life, darkness into light, the manger of our being, with passions tied like cattle and beasts, into the Divine Temple, into the glorious palace of never-ending rejoicing in Christ Jesus."
Again, in a powerful way, through this service we are reminded of the task we have now, to unite with the Holy Spirit, to find Theosis, and maintain it in our life constantly.