LTSpice is free (but not open source) and runs on OSX and Windows (and on Linux through Wine) and is pretty well featured. Its used pretty widely in industry and academia. I would strongly recommend it.
There is also some simulation capability built in (linked to?) KiCad using ngspice, which is free and open source. However, the feature set of ngspice is not very good. I would recommend using LTSpice.
LTSpice on Mac has an atrocious user interface that must have been designed by a clown. I have a Macbook pro and a WIndows PC and only ever run it under Windows, it's that bad.
To be fair, the Windows version of the UI sucks too. It has always been shitty.
The problem is that the Mac version of LTSpice is close to the Windows version but ... not completely there. The two versions share the same silly function-key user interface (F6 for copy, F7 for move, F8 for drag, F3 for wire, F5 for cut, see, i know them!), but there are some things missing. Like on the Mac you can't tell the program where to store the ephemeral files created during simulation. And there's no dialog for entering simulation commands; you have to add a Spice control text box and you have to look up the syntax for your analysis type. And yeah, after awhile you know the syntax, but why oh why is that dialog missing?
Then there's the whole thing about adding your own library parts.
I emailed them back in 2014 about it and Mike himself replied the following. A few years later, I went to an Analog Devices event and went to his talk about LTSpice. He seems to be a genius in maths and electronics, but not a good UX designer IMHO.
My email:
"Hi, Why does the LTSpice OS X version have a really outdated appearance compared to the windows version? It doesn't even have a proper toolbar. When do you plan on updating it with support for retina graphics as well? It looks like from the 1970s! Pretty hopeless!"
Response:
"On the PC version, I added many features over the course of 15 years
but never changed the software in the interest of not confusing
established users. In hindsight, this might have been a mistake.
For example, the tool bar was added as an after thought since people
in the 1990's thought that a GUI meant bitmaps. But skilled users
knew all along that everything was available by right mouse click
menus and moving the mouse all the way up to the top of the screen
just to have to move it back to the schematic area was a waste of
effort.
On the Mac Version, I removed the parts of the GUI which I never use.
I removed the toolbar from the Mac version because all it did on
the Windows version is teach people the wrong way to use the software.
Use the right click menus instead so you don't have to move your
mouse so much.
--Mike"