I saw both over think and overthink are used. Are they correct? Is overthink preferable or I can use no matter which one?
My first instinct was that over- is a prefix and would not normally be spelled separately. Here's what Dictionary.com has to say on the matter:
a prefixal use of over, preposition, adverb, or adjective, occurring in various senses in compounds (overboard; overcoat; overhang; overlap; overlord; overrun; overthrow), and especially employed, with the sense of “over the limit,” “to excess,” “too much,” “too,” to form verbs, adjectives, adverbs, and nouns (overact; overcapitalize; overcrowd; overfull; overmuch; oversupply; overweight), and many others, mostly self-explanatory: a hyphen, which is commonly absent from old or well-established formations, is sometimes used in new coinages or in any words whose component parts it may be desirable to set off distinctly.
And for overthink
verb -thinks, -thinking or -thought
to spend more time thinking about something than is necessary or productive
And as @Jason Bassford points out, the meaning could change if you separate them (in this, or other cases). So I recommend writing them together: overthink.