I picked up the Oz 75th Blu-Ray that was $3.96 on Black Friday, and there is indeed a mono track.
I just popped it in to verify and it's definitely there but it is oddly located in the Special Features section and is not accessible in the audio options portion of the menu. Just select it in the Special Features or switch to it on your remote but it's definitely there.
Also, it is not listed on the packaging either, so maybe this is what's causing confusion? My friend also has the 3-D box set that just came out at the same time and my disc is the same as disc 2 in that set.