I am trying to customize behaviour of Enter key in Normal mode in VIM.
Here is my .vimrc:
nmap o
You can't map in CLI Vim, no matter how hard you try, because Vim can't distinguish from .
You must find another mapping or stick with GVim/MacVim.
edit
Some terminal emulators, like iTerm.app or Terminal.app on Mac OS X, allow you to set up shortcuts to send specific characters sequences to the shell. If you have that possibility it may be worth a try but you'll quickly get used to a platform-specific gyzmo that can't be ported so, well… I don't really recommend it.