In my opinion, nothing is safe. Good is always abused for bad if the risk is worth it by criminals no matter what, there's so many different ways to do something that you cannot ever know 100% of something.
Windows is not perfect, OS X is not perfect, Linux is neither perfect. Every OS has its own advantages and disadvantages, it's own weaknesses which can be exploited, blah blah.
My personal opinion is that OS X is more adapted for a general novice than Windows in terms of security because it's a bit more difficult to get used to with more password enforcement requirements, more Settings to disable things and also reboot the session, etc... Compared to Windows which is relatively extremely simple to override. A few weeks ago someone on here was trying to get assistance on how to execute an OS X malware sample because it was ignoring the restriction changes and still continuing to block the execution as OS X security knew it to be bad and that it would cause harm - on Windows it would have been a simple Windows Defender white-list exclusion which takes a few clicks, not a circus of trying different bypass restriction scripts in the console and having to reboot for each single one of them before being able to test if it was successful or not. This doesn't change the fact that OS X is not full-proof, OS X malware exists and is out there and criminals also target OS X a huge amount. If you are on the OS X security area and analyse malware for it, you'll know a lot of malware is out there for OS X.
My personal opinion is also that a Chromebook is the right choice for a novice, average user. If I don't need to be using OS X or Windows, and only need to do some browsing or something a Chromebook can handle, then the best option would be for me to use one for those tasks. It's safer that way. It likely won't always be like this though... When criminals have more interest in Chromebooks and ChromeOS, they'll be clever with it and find ways to discover its flaws and exploit them one way or another. If not through exploiting the OS itself, find ways to cause damage and gain income at the same time and apply advanced social engineering techniques to make it effective.
I recently gained an interest in OS X Internals awhile back and have been continuing my research and development/reverse engineering on it, and I can tell you right now that techniques such as code injection and the alike are all very possible. For example, on Windows you have DLL injection techniques... DLLs are Portable Executable's and thus are not supported on OS X. However, on OS X you have something called a "dylib", which stands fro "dynamic library", and you can inject these dynamic libraries into other running software... Thus being the OS X equivalent of DLL injection on Windows. Code injection on Linux? Yeah, it can be done as well just as simply.
It's all a game which runs on a timer-based interval, the aim of the game is to see how quickly someone can do it and how long you can last. Something you believed to be full-proof or secure 10 years ago is likely not the same in your opinion today. While security evolves, the criminal market and vulnerability exploitation to defeat the evolution of security also evolves. It's always the same. When I was born (literally), malicious software was being distributed on Operating System's like Windows 2000... Typically for destruction purposes specifically and not about income generation. Fast forward 10 years? Ransomware had already been introduced and the criminal market was booming by criminals who wanted to generate income instead of only cause destruction. Fast forward a few more years? Now we have ransomware prevalent in the wild like never before, lots of adware, crypto-currency mining rising on the daily basis, and network-based attacks are becoming even more severe with services being targeted in DDoS attacks with HUGE bandwidth resource reduction. At the same time? We have AV vendors moving to the hyper-visor for virtualization to try and monitor more and be more difficult to exploit in terms of monitoring scope, Artificial Intelligence and Machine Learning becoming even more stronger, services which help prevent DDoS attacks and the alike trying to up their game as much as possible to protect against the evolution of attackers... Oh, what's that ringing in my ear? It's the sound of criminals trying to get one step ahead because good is trying to gain an advantage over the bad they were initially doing.
If I confused you then I apologise because my example is probably not the best for this but what I am trying to say is that good vs. bad is constantly evolving and when we finally get a head start or an advantage, the bad will find a weakness in it and force us to continue upping our game. It works vice-versa. And it will never ever stop until everyone who is bad and trying to generate income from computer science with malicious intent decides to stop, which is likely never ever going to happen.
Use whatever you want to use but remember that your own actions are important. Stay cautious - using OS X over Windows doesn't make you invincible, or for any OS, however it's advisable to use an OS with less usability which provides your needs as much as possible.