Could you please answer a few questions regarding this OS?

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- Does nvOC have a "graphic" online interface like SMOS?

Dont know SMOS, but it has a telegram notification and the web interface looks like this :

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- Is it possible to easy install new miners?

Almost all important miners are included, adding new ones is easy too

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- Is there any "hidden" fee for using it?

No fee at all for nvOC, some miners like claymore has their own fee

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- Does it have general "quality of life" features, like: restart with auto connect, auto restart miners, ...?

Yes, Auto restart miner and reboot rig if hash rate drops or gpu fails, timer reboot,...

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- Does it update (or auto update toggle) video cards / OS?

It has auto update toggle for stable and beta
System updates are blocked to get more stable mining and will enforce by nvOC

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- Is it possible to enforce overclocking for GPU as soon as the system restarts (auto)?

Yes you have many options for overclock and power. by individual GPU, by mining coins , all same ...