For a very long time I believed in OSAS. PotS was, in fact, one of the most difficult doctrine for me to grasp in Reformed Theology. However, if one thinks how God leads us home, I think there isn't the tension that exists in OSAS. Whereas once-saved-always-saved sound combative, almost insistent that we're saved; preservations of the saints is indicative that we simply rest in the palm of His hand to guide us to the promise land.
I would have to agree if it's looked at in terms of free will.
once-saved-always-saved sound combative,
Maybe that's why we always see those silly statements "that OSAS means you can go do whatever you want and it's okay".