What I emphasize about Old School for my game is:
3. I don’t railroad the players to ensure a particular plotline happens.
4. I don’t even railroad the players to ensure a particular dungeon is explored.
5. I don’t fudge dice.
6. I don’t “scale” encounters to ensure the party will have a challenge, nor to ensure that they will survive.
7. I don’t run modules. (To be strictly accurate, I don’t run modules without making drastic changes, saying “This is all wrong!” and ripping most of the contents out and putting in new stuff.) Because when I started running, we didn’t have published modules. And we liked it!
8. I don’t run published settings, I run a homebrew setting. Because when I started running, we didn’t have published settings. And we liked it!
9. I don’t have a problem with people playing unusual characters (as long as they fit the genre). I have a player who is running an intelligent, talking housecat (who is a former witch’s familiar), another who is running a magic-and-clockwork automaton (since my game is late Renaissance technology), a third who is running a wizard with a tentacle for his left arm, and a fourth who is a visitor from offworld, from a “Raygun Gothic” interstellar civilization.
10. My game is tactically transparent.