Pre-90s, I thought "Obi-Wan" was a name he was given in the Clone Wars, in honor of some great deeds he'd done helping save some alien world.

Kinda like...

Erwin Rommel: "The Desert Fox?... Now, that's a name I've not heard in a long time. A long time"

That's ^ just my own theory, or it could have been a military rank

The name on his birth certificate would have been "Ben Kenobi". That is a fact, proved by the two lines...

Luke Skywalker: "I'm here to rescue you. I've got your R2 unit. I'm here with Ben Kenobi"

Princess Leia: "Ben Kenobi? Where is he?"

Otherwise Leia would have said...

"Ben Kenobi? Wait what? Oooooh, do you mean Obi-Wan Kenobi? Right okay, I was confused for a moment there because my father never mentioned this guy ever being called Ben. Anyway, nevermind, shall we get on with escaping from this cellblock?"

Another thing Lucas totally contradicted/forgot for the Prequels.