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.

"Think over" means "ponder".

They have a very similar meaning and can often be used interchangeably. But "think over" means making a general objective analysis - a wholesome overview. While "think about" could be about any part of it, or the whole of it - there is no specification on this.

“Think over” means to give something (usually a choice) serious thought. And by that time, a consensus would've been reached. "I need some time to think over his proposal" To think about something means that you considered something. You don't have to be serious about that something, you can be spontaneous "I was just thinking about you when you called"