Natively on OS X? Yes, but given the restricted choice of DB versions, lack of support for critical/desirable products, and lack of certification/general flakiness/incompatibility with OS X upgrades, probably not too many people are.
Most people are probably running under virtualization using VMware Fusion, Parallels, or VirtualBox.
I'm currently running APEX 3.2.1 and 4.0.2 with Oracle 11g R2 EE on Oracle Enterprise Linux 5, and the Oracle XE 11g beta on Ubuntu 10.10 using VMware Fusion. Not production systems but perfect for R&D and training. You can do all sorts of things with VMs you wouldn't want to do with a real server.
I have vmware with Oracle Linux running RAC (just for fun) it making my notebook very slow, I have 4gb ram, need to bring up 1 node at a time, I deleted whole everything because it making my notebook very slow.
I like linux but I think Apple's team afford worth consideration, their desktop workspace in Lion is very creative and nicely done.
Using Oracle in Mac is making my work-life balance :)
Thanks for your tips
I have a macbook pro (2011 version) upgraded to 8Gb for this reason.
It's the absolute minimum for working comfortably with more than one virtual machine (and Oracle in particular - because the more RAM, the more memory caching, etc). It flies.
I even did not install a SSD and I am glad I didn't. It's fast now (and allows me to boost my laptop at some point in the future).