The phrase in question, he who began a good work from Philippians 1:6 is actually the last part of a sentence, which starts at verse 4.
(1:4) In all my prayers for all of you, I always pray with joy (1:5) because of your partnership in the gospel from the first day until now, (1:6) being confident of this, that he who began a good work in you will carry it on to completion until the day of Christ Jesus. (1:7) It is right for me to feel this way about all of you, since I have you in my heart; for whether I am in chains or defending and confirming the gospel, all of you share in God’s grace with me (NIV).
By going back to verses 4 and 5, it becomes apparent that the good work in v. 6 refers back to their partnership in the gospel (v. 5). In other words, the church at Philippi was partnered with Paul through their financial donations and,
therefore, shared in his fruit:
Moreover, as you Philippians know, in the early days of your acquaintance with the gospel, when I set out from Macedonia, not one church shared with me in the matter of giving and receiving, except you only; for even when I was in Thessalonica, you sent me aid again and again when I was in need. Not that I am looking for a gift, but I am looking for what may be credited to your account. I have received full payment and even more; I am amply supplied, now that I have received from Epaphroditus the gifts you sent. They are a fragrant offering, an acceptable sacrifice, pleasing to God. (Phil 4:15-18)
That was the good work which had begun in them and which Paul was confident God would carry on to completion—their partnership in the gospel.
Furthermore, verse 7 goes on to state that it was right for Paul to feel so confident about this because he (Paul) had them in his heart, meaning he would pray for them. When one examines the context of Phil. 1:6 it becomes clear that he who began a good work cannot be correctly used to support once saved always saved. Only people with a preconceived idea about this verse or who never considered the context will think it supports once saved always saved. The same can be said about John 10:28! Now you know what he who began a good work means and doesn't mean.