After making sure the right alt key is setup in the Terminal app as @junnu suggested, you can do the following to free up the right option key for the special characters you need:
(setq mac-right-option-modifier nil)
EDIT: Was a bit too quick, as I now read that you want to change the behaviour of the left option key...
Anyway, here is a suggestion: take a look at the variable
ns-alternate-modifier and its documentation (with
What is this variable's value in your current configuration?
EDIT 2: As pointed out by Stefan, this only affects the GUI setup. This setting is part of
ns-win.el and thus only loaded in a windowed environment.