For OSX there is now hopper, a binary debugger and reverse Engineering tool. Something between Olly and IDA with a nice interface. Can work on x86 as well as ARM binaries. Hopper is not open source.
http://www.hopperapp.com
I'm really really hoping Hopper can catch up to IDA. I'm still not good enough with assembly to make much use of either, but the free version of IDA can make some really nice flowcharts which are pretty helpful.