A 50

Shimmering armor, available 1 level before shadowdance. You can be OA-free for at least one attack per encounter with a rod of time distortion or a z'tal/serpent familiar. I suppose Sling Expertise, Drow Fighting Style or Thri-Kreen Shooter/Thrower counts too, if you can get the appropriate weapon type to be an implement for you.

That's all I see that's even close to reasonable. The halfling feat Nimble Spellcaster only works for area attacks, armor of night is paragon tier, and the belled branch symbol only works for divine powers.