How do "once saved, always saved" advocates explain the Parable about wise virgins? The Parable Matthew 25:1-13 King James Version (KJV)


  Then shall the kingdom of heaven be likened unto ten virgins, which
  took their lamps, and went forth to meet the bridegroom.
  
  2 And five of them were wise, and five were foolish.
  
  3 They that were foolish took their lamps, and took no oil with them:
  
  4 But the wise took oil in their vessels with their lamps.
  
  5 While the bridegroom tarried, they all slumbered and slept.
  
  6 And at midnight there was a cry made, Behold, the bridegroom cometh;
  go ye out to meet him.
  
  7 Then all those virgins arose, and trimmed their lamps.
  
  8 And the foolish said unto the wise, Give us of your oil; for our
  lamps are gone out.
  
  9 But the wise answered, saying, Not so; lest there be not enough for
  us and you: but go ye rather to them that sell, and buy for
  yourselves.
  
  10 And while they went to buy, the bridegroom came; and they that were
  ready went in with him to the marriage: and the door was shut.
  
  11 Afterward came also the other virgins, saying, Lord, Lord, open to
  us.
  
  12 But he answered and said, Verily I say unto you, I know you not.
  
  13 Watch therefore, for ye know neither the day nor the hour wherein
  the Son of man cometh.


Did the 5 foolish virgins were saved once but lost their salvation in the end? Clearly, they were waiting for Christ(the bridegroom) which means they were saved but didn't have faith(oil).
While it is impossible to account for all OSAS interpretations of of the passage, the most obvious answer is that the unwise virgins were not saved. Remember that certain (purported) proponents of OSAS allow that someone can be (self) deceived about their status as one of the elect. 

Consider what Calvin wrote: 


  Experience shows that the reprobate are sometimes affected in a way so similar to the elect that even in their own judgment there is no difference between them. Hence, it is not strange, that by the Apostle a taste of heavenly gifts, and by Christ himself a temporary faith is ascribed to them. Not that they truly perceive the power of spiritual grace and the sure light of faith; but the Lord, the better to convict them, and leave them without excuse, instills into their minds such a sense of goodness as can be felt without the Spirit of adoption .... there is a great resemblance and affinity between the elect of God and those who are impressed for a time with a fading faith .... Still it is correctly said, that the reprobate believe God to be propitious to them, inasmuch as they accept the gift of reconciliation, though confusedly and without due discernment; not that they are partakers of the same faith or regeneration with the children of God; but because, under a covering of hypocrisy they seem to have a principle of faith in common with them. Nor do I even deny that God illumines their mind to this extent .... there is nothing inconsistent in this with the fact of his enlightening some with a present sense of grace, which afterwards proves evanescent (Institutes, 3.2.11).


The unwise virgins believe that are ready only to learn too late that they lack the needed oil (grace?).