Yes! I am going through the exact same thing with my 18 month old son, except that my son does try to imitate some of the words we tell him.
I think what we are going through is quite common from other moms who I have talked to who have sons the same age.
Boys typically develop language skills later than girls.
My son seems to know many, many, words but it is quite difficult to understand what he is saying sometimes becasue he will only say the first syllable of the word. And he seems to understand everything I say to him.
He still does not say the word "NO" but will turn his head from side to side when he doesn't want to do something. When I ask him to say the word "No", he will just say "NNNNNN"and end with a scream, he can't really get the "O" out.
He also says "SA" for stars and squirrels. He says "MAYMA" for mailman but a lot of what he says is indecipherable.
I wouldn't worry.......