I would recommend the Knock! magazine line, which includes articles, adventures, monsters, etc. They're OSR, not OSE, but the distance between the two is small.
Brad Kerr has recently been hired as an in-house adventure designer by Necrotic Gnome, but he's an award-winning adventure writer on his own. One of his is nominated for an Ennie this year.
The Monster Overhaul, which again is OSR, not OSE, but can be used with it without issue, is absolutely the best bestiary of all time. It is amazing.
Obligatory plugs for The Stygian Library, Into the Wyrd & Wild, any of the Thousand, Thousand Islands books if you can find them, and Tomb of the Serpent Kings.