Companies like Elektros sell OPV kits for the Oscar II that can easily install if you are just a little bit handy. https://www.elektros.it (No affiliation).
I installed another valve (an expansion valve found at my espresso machine repair shop) and added the tubing to direct the water into the pump intake.
It was quite easy to do (and fun!) but after having adjusted the pressure to 9 bar at the group head with a manometer on a portafilter (with a filter!), there was a weird sound coming probably from the valve. Edit: I found this awesome article (http://coffeetime.wdfiles.com/local--files/opv-over-pressure-valve/OPV_Valve.pdf) from @DavecUK and I think the noise in hearing is temporary. Also I may need to look at the valve spec since it is quite small (similar to this one but with a nut to adjust and it is written 12 bar on it since it is already adjusted at 12 bar I guess! - https://coffeeaddicts.ca/products/nuova-simonelli-expansion-valve-1) but it is all brass should be of quality but it’s a straight one that I had to add a right angle fitting
I’ll have to dig a little bit since the expansion valve was one marked 12 bar on it so maybe it doesn’t like being adjusted to 9 bar. Anyone has any thoughts on that?
But all in all, it’s an easy mod and I’m sure with the proper valve (or adjustment) it will be a nice addition.
I feel like my coffee have more sweetness post the “silent” pump and the pressure limit at 9 bar installation but I’ll need to run more test to confirm.
Since I’m looking for another machine, I can comment that so far the Espressos from the MaraX with the same coffee never had the little acidity that I get from the Oscar II. The acidity seems a little bit tamed post modifications but the sweetness really seems to be the improvement. But note that I’m no taste expert neither objectives since I didn’t do blind tests.
Annex ( :)): I installed the valve right after the pump and redirected to the water tank out / pump intake like Elektros are suggesting to do with their kit. Their valve seems a bigger but maybe this is only because it includes the right angle and the pass through. In my case à added the right angle and pass through to a straight valve.