No it's not. RAM disks are kind of a thing of days gone by. Mac OS used to have it as a built in feature in some pre-OSX versions. Basically, you would put a ton of RAM in your machine and then create a RAM disk, then you would place your application and the files you wanted to manipulate in the RAM disk and it would be lightning fast. This was before SSD's, of course. And, because the hard drives were much less dense (for example I had a 9GB Hard Drive in my last OS 9 machine that was actually BIGGER than a standard 3.5" desktop hard drive), they were very very slow.
Not much use for them today, I just wanted to experiment with it. The RAM disks is deleted every time you turn your computer off, and must be re-created/re-mounted each time. Once RAM loses electricity, all of the data on it is lost. It's not like a Hard drive or a flash storage drive (like an SSD) which stores it's information even when it's switched off. That means the OS must first boot, create the RAM disk, and then load files into it.
There ARE some Linux distributions that are designed to be very small (the entire OS being only a couple hundred megs, or less). They run as a bootable LiveCD. Basically, you pop the CD in, and it loads the files from the CD directly into the RAM. The entire OS runs completely in your RAM. Apart from the initial load time, it is INSANELY fast.
I like Macpup:
http://macpup.org. It's based off of puppy linux, but has a more polished, almost OSX like interface. It's free, just download it and burn it to a CD or make a bootable thumb drive, fire it up and see what you think.
I don't know that there are a lot of practical purposes to it, but if you make a good sized thumb drive into a puppy linux bootable drive, you could install software on there and have a workable OS. Just remember, you want a pretty good amount of RAM. You can still run applications stored elsewhere, but only applications loaded with the OS into RAM will be that fast. In addition to the RAM taken up by the OS and applications, and all of the files, you of course need RAM to run the computer!
In theory, OSX COULD do this. BUT, A) It would need to be written with a 'bootloader' of sorts that would load all of the files into RAM. and B) You would need a TON of RAM.
There aren't a lot of honest uses for this, although in the enterprise world entire clusters of servers with tremendous amounts of RAM that run their operating systems and all of their files inside the RAM do exist. Situations where insane performance is necessary. But it's not cost effective. Fun to play with though!
But anyway, a RAM disk, is loaded inside the operating system. So no, an OS cannot be installed on a RAM disk. However, there are OS'es that run inside RAM completely! (Meaning they do not store their files in the hard drive, but rather in the RAM, like MacPup!)
I may fire up MacPup and take a video or something, it's kind of fun to play with! More of a tinker toy, but still tool! (With time, patience, and the right knowledge it can be a very powerful OS, but with specific uses in mind. There are better linux distros for the 'everyman', but those don't run entirely in RAM!)
Edit: As it turns out, MacPup doesn't work (out of the box at least) with the MacBook. Some have gotten it to work but none of them (that I've found) mind sharing with the rest of us how they did it.
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