'Over' is already an adverb. If you want to modify a word with 'over', combine the two words or use a hyphen, as in 'over-concise'. Although 'overly' is now common usage, it is still looked down upon by the literati.

There is nothing wrong with using dashes, but if you do, use the correct dash. The em dash is U+2014. If you cannot type an em dash, the custom is to use two en dashes.