ouija rated a (??) based on this:

[Origin uncertain.
The word has been variously explained as (a) < French oui OUI adv. + German ja yes (see YEA adv.), (b) < an ancient Egyptian word for ‘good luck’ (although app. no such word exists), and (c) < the name of Oujda, the name of a city in Morocco.]

this is from the revised draft edition of OED © 2004;
the 2nd edition has only this: [f. F. oui yes + G. ja yes.]

so they've backed off from the oui/ja explanation.