For me "Old School" means D&D.
White Box D&D.
*Before* The addition of Greyhawk and its namby-pamby D4, and Thief class.
Wanna do something? Roll a D6. Whaddaya mean, what do I need to roll? Just do it! *I'll* tell you what happens! To slow, you're dead, roll a new character!
All the people using the term around me seem to be trying to lose all the rules, polydice etc and get back to the D6-or-nuthin' days, when the dreadfully misleadingly named "1st edition" was still a gleam Mr Gygax's mind's eye.