The London-based firm has signed a binding agreement with Astorg according to three people with knowledge of the situation who spoke to Reuters.
TDR Capital and Lion Capital were previously reported to be interested in the asset, while CVC Capital also made a bid, the people said.
Astorg is set to make a substantial gain on the business it bought for an enterprise value of just over €300m in October 2007, having grown it to have an EBITDA of about €100m since.
That figure was expected see the company sell for between €800m and €1bn including debt.
Paris-headquartered Astorg put OGF on the block in April by hiring Goldman Sachs to fund a buyer.
It made the initial investment through its mid-market Astorg IV fund, which it closed on just over €800m in June 2008.
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