The OED does not classify it as rare in the sense most often encountered nowadays:You will find it in OED. It is classified as 'rare', so maybe it's a verb having a second childhood!
3. trans. and intr. To consider or ponder too hard or too seriously; to think too much about.
Is it perhaps under the entry for "over," in a list of words using that prefix which do not have their own separate entries?It is not in my Webster's unabridged International Dictionary, but I have seen it used many times, and I have used it myself, to mean to dwell on too much.