I know you're looking for a comparison of the "new" syntax of zsh and oil but I think it's worth noting the biggest difference is probably going to be Zsh is an extended sh shell with a bash (well ksh) emulation mode capability whereas oil is a shell where the language is a superset of bash syntax.
Zsh tries to be similar to bash but it has no problem not aiming to be 100% bash in normal mode. Osh wants you to be able to replace #!/bin/bash with #!/bin/osh and run your script without setting the mode to something special. Then when you're ready to extend it you can write either bash or osh without toggling emulation mode or worrying about which syntax or behavior differences you'll run into not being in emulation mode.
