In Mac OS, what is the keyboard shortcut to switch between windows of the same application?

The default shortcut is Cmd` (that's a backtick).

You can always change that shortcut if the given one does not work out for you. Go to System Preferences → Keyboard → Keyboard Shortcuts.

Here, assign a custom combination to

• Keyboard » Move focus to next window (under OS X 10.9 and above)

  

• Keyboard » Move focus to the next window in application (until OS X 10.8)

  

This will allow you to toggle between the open windows of any application.

Note

Cmd` works only if all windows are on the same workspace (comment @thias)

command backtick

⌘ + `

This is what I found on my default US english keyboard setting.

1. Toggle between Different Apps -> Command + Tab

2. Toggle between Same App in Same Workspace -> Command + `

3. Toggle between Same App across different Workspace:

   1. View same App windows across All Workspaces -> Control + Down Arrow
   2. Then select with arrow keys or click the app window you want.