TLDR: They are nearly identical in weight. Dimensions are identical (except for the gap width). The MC feels lighter in play and has more ease in rapid redirects and acceleration. The MCMO is noticeably more powerful and stable while still maintaining a nice balance between control and stability. As a trade-off, it does feel a bit heftier in play and is slightly more reluctant to rapid changes than the MC. The wider gap width in the MCMO is an upgrade for me - my custom nylon strings would sometimes snag in the MC where as the MCMO binds perfectly and produces really clean throws. I love both but the MCMO is now my favourite.
Background: The Markmont. Classic (MC) had long been on my list of yoyos that I wanted to try. Last January (2020), I finally indulged when Mark released some more of his self-branded MCs and I ordered one in the gorgeous Street Penny colourway. When I first threw the MC, I knew it was something special and unique. However, I didnāt āconnectā with it initially because it was so different from most of the yoyos in my rotation at the time.
Fast forward to Fall 2020, I was going through an intense job application for a university professor position and was super stressed. The MC started calling to me and it became my main throw during this time. It feels so comforting in the hand, it is so quick, nimble and light on the string. It helped me find a state of flow and calm while throwing. What I find most distinctive about the MC is its ability to rapidly change direction and speed, something that obviously lends well to Markās style of play.
Having submitted a beast of an application (I have since been shortlisted and had an interview - Iām now waiting to hear if Iām advancing to the final stage of selection), I decided that I had earned a level-up and splurged for a Markmont. Classic Magnum Opus (MCMO) in the limited prototype colourway, Cobalt King.
Whatās the big deal: For those who may not know, the MCMO is a redesigned version of the MC, using 7075 aluminum alloy instead of 6061. The MC is Mark Montgomery (@Markmont)ās brainchild. Originally released under One Drop Yoyos (OD) banner in 2014, Mark has since adopted his own brand and brought the MC with him - however he still continues to work closely with OD.
One Drop is a company known for redesigning some of their earlier designs to take advantage of the unique properties of 7075 aluminum (higher strength and density) to provide a yoyo that is similar to the original design but altered with a tweaked weight distribution that provides a new throwing experience. Despite the higher density of 7075, these versions can actually be similar in weight or lighter than the original, and will often have more rim weight. OD refers to this procedure as āboostedā and therefore the MCMO could be thought of as a āboostedā version of the MC. Other examples of boosted OD models are the Kuntosh 5000qv, Seven Summits, Legendary Terrarian, and the Nitro. In my opinion, optimizing designs to make the most out of the unique properties of 7075 aluminum is a signature specialty of One Drop. They do it very well.
So years after the release of the MC, Mark was finally ready to consider doing some tweaks to this beloved yoyo. He was wise to trust in the mastery of 7075 that the guys at OD have honed over these yearsā¦
Similarities: My MC and MCMO are identically set up with spike side effects and deshielded OD 10-ball bearings. On the scale, they are nearly identical in weight - the MCMO is 0.12g lighter.
The dimensions are identical, except for the gap width (discussed below). They look almost identical with one visual difference, the MC has a flat angled cut around the flat base of the cup, where as the MCMO has a slightly grooved cut. This helps differentiate the two models.
In play: This is where these yoyos differ, not hugely but it is definitely noticeable to me. The MC feels lighter in play and has more ease in rapid redirects and acceleration. The MCMO is noticeably more powerful and stable while still maintaining that signature balance between control and stability. As a trade-off, it does feel a bit heftier in play and is slightly more reluctant to change speed and direction than the MC.
Gap Width: The MCMO was also redesigned with a wider gap width - MCMO: 4.62 mm; MC: 4.32 mm (as reported by YoyoExpert). The wider gap width in the MCMO is an upgrade for me because my custom nylon strings would sometimes snag in the MC where as the MCMO binds perfectly and produces really clean throws.
Note on āsmoothnessā: I have heard other players describe the MCMO as smoother than the MC. Iām not sure exactly what they are referring to. I will say that my MCMO spins very smoothly and, though I have only taken it apart twice, has been easier to tune than my other OD side effect yoyos, including the MC. It is really smooth, bordering on glass smooth - both on the fingernail and on the string. It is also super quiet and it came with a quietest OD bearing that I have ever used. But importantly, in my opinion, the wider gap width helps with cleaner releases (less wobble) when the yoyo hits the base of the string. The MCMO throws very cleanly/smoothly and transitions so well between trick elements. It is a true joy to play.
The finish: Both yoyos have ODās signature pyramatte finish. I love this finish, even though some blasts perform better for grinds, the pyramatte is so smooth and silky to the touch. It looks awesome, like a very fine stone wash finish. As discussed in this article, 7075 aluminum anodizes differently from 6061. So while both yoyos have the same finish process, the finish on my MC is more matte and silky where as the MCMO is a bit shinier and slightly less silky. Markmont claims that has an additional advantage of further reducing string friction.
(Note: Iām hoping to do another run of BearHugs with some new colours when I have the time. The standard collar size fits the MC and MCMO perfectly.)
So in conclusion , the two yoyos are obviously very similar as intended. The biggest noticeable differences are in the way that they play. Overall, I love both but my heart has now been stolen by the MCMO. It keeps almost all of the essence of the MC, but pushes the feel and performance slightly more to where I want it. Well done @Markmont and @The_Machinist, you have yet again created something special. The Markmont. Classic Magnum Opus is a yoyo that truly lives up to its name! 
Let me know if you have any specific questions that I didnāt address. I hope that this review is helpful for anyone who is as fascinated by OneDropās magical powers as I am. 
Edits: Additional details added to highlight that this yoyo is a Markmont. product.
I didnāt end up getting that position I interviewed for but it was a good experience for me. Just started a postdoc at a university that was much closer, so life is goin alright.