From your first picture of your Oracle frame it seems that it’s not at its greatest width where the tire passes. Maybe it’s just an effect of the point of view the picture has been taken from. If not, it’s a bit of a shame, since a different (rounder) bend of the frame or slightly longer legs would probably cure the issue.
It also depends on the rim width. Nimbus Dominator 2 rim is 33 mm inside, whereas Mad4One rim is 38.8. The tire should then be a bit wider on Mad4One rims than Nimbus rims… The clearance you get on a Nimbus frame should not exist on a Mad4One M frame 
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That assumes the widest point of the tyre always aligns with the widest part of the frame. The frame is bowed, so only 1 section is 92mm wide.
The 27.5x3.25 Duro Crux does fit in the M sized M41 27.5 frame, but the rubber stills rubs as a section of the tyre, the nobbly bits, are not at the widest point on the frame.
Sure, get the L frame if you can, I may have in hindsight, but the 27.5x3.25 Duro Crux now happily fits my M frame M41 27.5 with just a small amount of help from my side cutters. Easy fix.
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You are correct. Once I typed my response, and realized the same thing you did, I also realized that the same thing might apply to a M4O medium frame.
It is nice to know/learn that a pair of side-cutters is all that’s needed to snip the side tread knobbies a bit to create clearance.