It is saying that it is impossible for a believer who had once been enlightened by the spirit and fall away to come back.
Heb 6:4 As for those who at one time saw the light, tasting the good things from heaven, and having their part in the Holy Spirit,
Heb 6:5 With knowledge of the good word of God, and of the powers of the coming time,
Heb 6:6 And then let themselves be turned away, it is not possible for their hearts to be made new a second time; because they themselves put the Son of God on the cross again, openly shaming him.
Read it properly. It is not saying that it is impossible to turn away but it is saying it is impossible to turn away and return.
(RV)
Heb 6:4 For as touching those who were once enlightened and tasted of the heavenly gift, and were made partakers of the Holy Ghost,
Heb 6:5 and tasted the good word of God, and the powers of the age to come,
Heb 6:6 and then fell away, it is impossible to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
(ISV)
Heb 6:4 For it is impossible to keep on restoring to repentance time and again people who have once been enlightened, who have tasted the heavenly gift, who have become sharers of the Holy Spirit,
Heb 6:5 who have tasted the goodness of God's word and the powers of the coming age,
Heb 6:6 and who have fallen away, as long as they continue to crucify to themselves the Son of God and to expose him to public ridicule.
(ESV)
Heb 6:4 For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit,
Heb 6:5 and have tasted the goodness of the word of God and the powers of the age to come,
Heb 6:6 and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt.
The original greek literally says "and falling aside". How is this used to support OSAS?