Over all, a phrasal adverb, means above everything; overall, as an adverb and adjective, means in all parts or taking everything into account. Neither over all nor overall improves most writing: over all rarely adds to what over already handles well, and overall only adds superfluous emphasis that should be done without. Prefer I like him better to I like him better overall and He shot the arrow over their heads to He shot the arrow over all their heads.
In rare cases, over all may better suit the rhythmic poetry of a sentence than over by itself. Both The mountain loomed over the land and The mountain loomed over all the land mean the same, but where the former places rhythmic emphasis on loomed (and, in turn, on the subject of the sentence), the latter shifts the emphatic burden to all (and the object of the sentence). Though all is less colorful than loomed, a writer may, on select occasions, include the word to alter the rhythm and, as in the above example, shift the emphasis from subject to object.