Is there an equivalent to DBus on OSX?

If you need to use DBus you can install it via Homebrew.
Homebrew is a package manager for OS X providing a lot of libraries which where mostly written with Linux as their primary target.The DBus package can be found here

However, this is not installed by default and if you intend to write Cocoa applications the way to go would be to use the (distributed) notification center as mipadi explained in his post (which you also accepted).

If you need a robust middleware for communication you can also have a look at IceTouch from ZeroC which is a Cocoa library for using the services and functionality offered by their Ice framework.The package contains IceStorm for example which is a publish/subscribe server or the IceGrid service which can be used to build large scale distributed systems

Probably the closest analog would be distributed notifications (or notifications for same-process communication). It's not a perfect analog, but it fits into roughly the same niche. You can read more about it in Apple's Notification Programming Topics, particularly the part about the distributed notification center.

There's Mach's ports, which will probably do what you need: http://en.wikipedia.org/wiki/Mach_(kernel) .