You have to consider also, Gary Gygax and a lot of the other "geniuses" (yeah right) responsible for D&D OA did not do as much -though they did do a considerable amount- research as they could/should have done before the book was released. And the book was targeted at gamers who wanted more Ninjas and Martial arts in their D&D Games. So the process of determining what to call the OA wizards went like this
GARY: "Ok, we've got Ninjas, we've got some martial arts, what else do we need to put into this book?"
Genius 1: "Whatever you say Gary."
Genius 2: "Looks good to us Gary."
Gary: "Well, yeah but maybe we should have a few more classes, like figther types, theives and Barbarians."
Genius 3: "That's a great Idea Gary!"
Genius 4: "You're a game designing god Gary!"
Gary: "Yeah, right ok, well then what are we gonna call these classes? 'Bushido' roughly translates into 'way of the warrior', so let's just call the fighters Do's."
Genius 1: "Sounds good to us Gary."
Genius 3: "We'll send it off to publishing right away!"
Gary: "No, wait. What the hell am I thinking? 'Do' means 'Way' or 'Way of', so we call the fighters Bushi, I guess we'll just call the Barbarians...um...Barbarians, gotta have Samurai of Course and then I guess we'll have the wizards called...uh...I dunno, how do you say Wizard or something like wizzard in Japanese or chinese?"
Genius 4: "Whatever you like Gary."
Genius 3: "You da'man Gary!"
Gary: "I think it's Wu-Jin."
Genius 1: "How do you spell that?"
Gary: "I don't care, I was up all night with my gaming group last night, just like every other night of My life since I finished reading Lord of The Rings."
Genius 1: "Don't worry Gary, We'll take care of it."
Genius 2: "Yeah you go get some sleep."
-Gary Leaves-
Genius 3: "So how do we spell it?"
Genius 4: "Here, W-U-Space-J-E-N...there send that off, and have them come up with a few new spells but incorporate most of the old spells from the players handbook."
I could be wrong, but I'm pretty sure that's how it went.