I've benefitted many times from the knowledge Ive found on this forum, so this finally seems to be a topic that I can contribute to with some level of first hand experience. To answer the original question:
NO, we've found that you definitely don't need an adjustable gas block to run an OSS on your rifle. In fact, the Larue Tactical rifles we have with OSS mounted on them run best with the gas block on the "unsuppressed" setting.
As previous guys have mentioned, there is most definitely a weight difference between flush mount OSS and others. Its very noticeable when you have both an OSS and say the Omega or Sandman in hand, but the weight difference seems to make less of an impact once its mounted on a rifle - and thats the consistent feedback we get from the LE evals we've conducted. Like many other companies, it clearly took OSS a while to find their footing and get their R&D settled on their current product line...we know this from experience during the painful days of being a dealer 2-3 years ago. Their new models (Helix and Elite 556, etc) are very similar in size to other suppressors in that space and I can add some comparison pics below.
As far as suppression level, its very subjective with every suppressor/rifle combination. You can find a quieter can on your AR than the OSS if decibel reduction is number one above all, However we've found when suppressing rifles things like blowback, ease of use, maintenance, customer service, etc all get thrown into the mix - it just really depends on what the end user feels are their priorities. Mostly we find that evaluators agree that the "tone" is different between all the cans, but the difference in sound suppression isn't that great between them.
For a little context on how I've learned this: We run a small vet-owned business selling suppressors/optics to LE departments and select private customers (basically meaning that we don't do gun shows, we don't have a store front, etc). This is a side business for us, so the challenge is to be very focused and deliberate in what we can offer, and know who our sales targets are. We don't do tactical training - that market is very saturated and well represented now - so instead we conduct live-fire evals as a way for customers to make up their own minds on what optics and suppressors (and sometimes rifles) fit their specific needs. For suppressors We rep OSS, SilencerCo, Dead Air, and sell just about every other make/brand...so its not like we think OSS is the answer for everyone. We have 18-20 rifles (DD and BCM mainly, but also HK, LMT, Larue Tactical, and others), each outfitted with suppressors and different optics for our evals - about 10 of these have OSS suppressors on them, and the balance have DAAs, SilencerCos, or Surefires. We are big fans of the Omega, and recently pair it with the DAA kymo quick detach instead of the direct thread mount. DAA suppressors will be on 4 of our guns later this month (we have one right now).
This gives our departments the ability to see first hand what works on similar platforms during live fires. We run all our rifles stock from the factory - except for adding some SSA triggers - so that we can demo how each suppressor works at baseline. Many guys add Adjustable gas blocks, suppressor optimized BCGs, etc to their guns to get them running like they want suppressed, but we feel that should all be factored into the decision process.
For example, we are working right now with an LE department who is purchasing suppressors for their entry teams and optics/suppressors for their sniper teams. We wouldn't necessarily recommend OSS for any Sniper rifle application, because those tend to be long barreled guns already that are used in single or limited engagements. There are a LOT of good options which are lighter for that type of rifle. The SBRs that the entry teams run with are another story. Size, Weight, Suppression, Blowback, Recoil, etc all get thrown into the equation here, and OSS does very well in this application with the departments we have worked with.
I apologize for the wordy response to a very simple direct question, but I thought it best to elaborate somewhat on how we came to hold these opinions. Clearly I'm not the only source of info on this topic or even OSS suppressors...we just have been in a position to see evals first hand from LE departments over the past year and its made an impression.
As I mentioned earlier, below are some pics (hopefully they show up):
Elite 556s mounted on BCMs (11.5" and 14.5")
ON RIFLE: OSS Elite 556 mounted on a DD MK18.
ABOVE: OSS Gen5 762 IFM7
BELOW: SilencerCo OMEGA with a flat 556 end cap instead of the 762 brake it ships with (I thought I'd show a 556 OSS vs the Omega as set up for 556. The brake the Omega ships with extends the length a couple inches or so)
OSS HX762 on an LMT
OSS IFM7 on an M1 Socom. This Socom is a good example of where I don't like the OSS really - too heavy and the thing feels like its 4 feet long. Im going to try an Omega or Sandman on it at some point.
PM or email me if you want any other details - hope this helps.