The author of The Outsiders never tells the readers who the outsiders are/were. Who do you think the outsiders are?

I can only answer this question as my own opinion since it is not really a factual question.

That being said, I believe the Greasers were meant to be the Outsiders in the novel.  When we think of the word "outsider", what comes to mind?  Those who do not fit in with the majority and the norm, those that are different.  In this novel, the Socs' would have been considered the normal, typical teenagers in the area.  They all came from money, had all the nice cars, popular/stylish clothing, good looks and played varsity sports. They were the popular kids in school.

The Greasers were the complete opposites.  They had very little money, old and worn clothing, used their two legs as transportation and were mostly looked down upon and ignored, except amongst each other.