The Chida, in his Sefer Pnei David, says that they must have been the descendant of Og, since Shamchazai and Azael died in the flood.

The Chida, in his Sefer Pnei David, says that they must have been the descendant of Og, since Shamchazai and Azael died in the flood.

Ibn Ezra (Bereshit 6:4) says that the Nephilim were around after the flood, but it looks like he also says that Nephilim weren't angels.