Makes lots of sense. What if my offline device gets broken, or burnt, or w/e? Is there a super safe way to store bitcoins? I'm getting to the idea there is no 100% safe way to do it. My online wallet could get hacked and my offline USB/HD could crack...
OS does matter to be honest . Using Linux would be better then using Windows because most of viruses out there are written for Windows.
So you could increase your safety by using Linux .
I would suggest you to use Electrum as a wallet , because It gives you a 12 words seed when you setup the wallet for the first time , and you can recover your Bitcoins & wallet with those words in a case of a computer failure or device get burnt as you said etc ... So just keep those words in a vault in a piece of paper or whatever . and you should be fine
~ Madness
If you don't know anything about Linux, then you shouldn't use it as you're more likely to configure something wrong, install something malicious or not update it properly. A lot of Bitcoin stealing malware have variants for Linux, the "less viruses" thing doesn't really matter when you are specifically being targeted by hackers. It's best to use the OS you are most comfortable with.
While were on this topic, is a MAC any safer than a PC?
Not really. Most modern OS's have similar security features.