Good day,
so creating a proper logo for Mac OS was actually harder than anticipated, as Apple in fact even holds a trademark on the font (!) they used in their classic Mac OS logo. Imitating that logo was out of the question. The same goes for the classic “apple” which everyone associates with the trade, as a German café actually got sued over using a red Apple with a child on it, as their logo. Even though the two were completely different, a seas and desist was issued.
So I was looking through the icons on my Mac, trying to find something, which people would associate with OS X, Macs or Apple immediately, though is so simple that not even after German law, could it be trademarked. I quickly found the logo for “OS X El Capitan”, the current version of OS X, to fit perfectly, as it is a simple coloured X. Since this is a normal, widely used, geometric shape, there seem to be no legal problems with using it.
Here it is: 
I may of course change the colour pellet on the X or the background colour, if you want me to, though I have to say, that I actually chose to make it this way, to distance the arrangement further from the original.
Have a nice day,
Ego