One major other use of の I know is in sentence endings. 1) かれは そう いった 2) かれは そう いったのだ。 3) かれは そう いったんだ。 #1 is direct and plain. のだ in #2 makes the sentence smoother, giving it a feel of a statement, like "It is that...". This のだ is as often as not abbreviated to んだ as in #3. You see these forms all over the place in Japanese texts.