'98 Plymouth Neon leaking greenish fluid...?

a greenish fluid would be anti-freeze from your radiator. a loss of anti-freeze will cause your car to overheat because there is little or no coolant in your radiator. not sure how an oil change could affect the radiator... I've changed oil on several different vehicles multiple times and have not had that problem. but pour some water or anti-freeze in your radiator and then watch and see if it is leaking from somewhere. if it is you should take it to a shop and let them replace or patch the radiator.