Yes, OVERDRESSING is a Scrabble word!

Letter values in Scrabble US: O 1, V 4, E 1, R 1, D 2, R 1, E 1, S 1, S 1, I 1, N 1, G 2.

Is it a valid word?

OVERDRESSING is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

Also: dress up, fig out, fig up, deck up, gussy up, fancy up, trick up, deck out, trick out, get up, rig out, tog up, tog out.

Not playable in Scrabble US: OVERCLOTHE.