at tænke om means to think of/about in the sense of having an opinion about something.

• What do you think about our new proposal? = Hvad tænker(/synes) du om vores nye forslag.

• What do you think about him? = Hvad tænker(/synes) du om ham?

at tænke på means to think of/about in the sense of having something on one's mind or considering something

• How often do you think about your ex? = Hvor ofte tænker du på din eks?

• I'm thinking about going for a walk tonight. = Jeg tænker på at gå en tur i aften.

• He always thinks about food. = Han tænker altid på mad.

That basically means "believe", and is also used for "think" in the sense of "have an opinion" ("I think that she is pretty") -- but not when "think" means "ponder in your thoughts" ("you'll have to think of an answer quickly!" or "she is thinking of me").