Q:

What is a free & open source alternative to IDA Pro for MacOS? 

I am looking for a free & open source alternative to IDA Pro runs on MacOS - the suggestions should have as close to the features of IDA as possible.  I should also be able to edit an executable that I am debugging (i.e. change/remove things). 

A:

I highly recommend radare2 which seems to suit you the best:

radare2 is an open source framework for reverse engineering and
  binary analysis which implements a rich command line interface for
  disassembling, analyzing data, patching binaries, comparing data,
  searching, replacing, visualizing and more.  It has great scripting
  capabilities, it runs on all major platforms (GNU/Linux, Windows
  *BSD, iOS, OSX, Solaris…) and it supports tons of architectures and file formats. [...]  radare2 is absolutely libre. [ Source]

You can find most of the features you have in IDA in radare (including the option to edit a binary that you debug) and in case you lack some feature you can always open a request on the Github page or develop it by yourself.  Note that, in my opinion, the learning curve of r2 is pretty steep and it'll take some time to get use to it. 
Installation
It’s recommended to use the current git version of radare2. 
Simply run:
git clone https://github.com/radare/radare2.git
cd radare2
./sys/install.sh

You'll need to perform several more steps in order to sign radare and make it ready to debug applications without the need of root permissions.   
Beginners guides 

Radare2 Book
Radare2 Explorations
Radare Demystified (Talk)

This should be enough in order to learn the basics of radare.   
User Interface
radare2 has a powerful command line visual modes to help you go through the program and super useful while debugging.  This is how the Visual Graph Mode looks like:   
 
The official graphic user interface of radare2 is called Cutter (originally named Iaito).  Cutter is built on top of Qt and C++. 

A:

This is not free and open source but Hopper Disassembler is $99US for personal use which compared to IDA Pro is super cheap.  There is a free trial version if you would like to check it out.  https://www.hopperapp.com/

