If you "think (something) over", normally it involves making a decision about a specific proposition.
"Think about (something)" can have that same meaning, but it can also be used in other ways. When you give your opinion on something, you're saying what you "think about (it), and you can use "think about" plus a noun, pronoun or gerund when you're describing what you're pondering, contemplating, considering, etc.-- in other words, what occupies your thoughts.