Ok, after some research, here's what I could find on "The Wizard Of Oz":
LD: Some (if not all) LaserDiscs are mono (like the 50th Anniversary Edition), others were fake (credit for info: skyjedi2005) stereo (like a Japanese release with reference number G98F5545).
link: http://www.lddb.com/search.php?search=wizard+of+oz&sort=title
DVD: Dolby Digital 5.1 for the simple release, original slightly altered (credit for info: SilverWook) mono included in the Collector's Edition and the 70th Anniversary Edition.
link: http://dvdcompare.net/comparisons/film.php?fid=114
BD: Both Dolby TrueHD 5.1 and mono sound is available on all official Blu-rays.