Are OS X installs specific to a machine type?
Yes and no.

In regards to retail installation discs / upgrades "purchased" from the Mac App Store, no, they are typically not machine specific.  These are general builds that are meant to install on any supported hardware.

Machine specific builds DO exist although at this point of time they aren't an issue for most people.  When Macs shipped with physical restore media (multiple CDs/DVDs) that media was very specific to that particular generation/model of Mac.  What this meant was that you could have an early 2008 MacBook and an early 2008 MacBook Pro, the MacBook media would display an error when booting the MacBook Pro and vice versa.

In regards to the actual installed files, yes, these days the installations are pretty generic.  I don't want to say that the installed files on a MacBook Pro Retina are the exact same as those on a MacBook Air, but for the most part this will be true.  In the past (early/mid PowerPC days) there were some differences but even then you could usually use one Mac to boot another.  The main problem we used to run into was remotely installing Tiger or Leopard using a mix of Intel and PowerPC Macs.  In those situations it was best to use PowerPC to PowerPC or Intel to Intel.

The only time this may not hold true is if you're trying to boot a newer Mac using the disk of an older Mac that has an older (older than the system the newer Mac shipped with) system installed.  For instance, if a new iMac was released tomorrow and I tried to boot it using 10.9.1 installed on my MacBook Pro Retina there's a good chance that it wouldn't boot because the iMac requires 10.9.3 or above.