Yes, OLDEST is a Scrabble word!

Letter values in Scrabble US: O 1, L 1, D 2, E 1, S 1, T 1.

Is it a valid word?

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

(used especially of persons) having lived for a relatively long time or attained a specific age
of long duration; not new
(used for emphasis) very familiar
skilled through long experience
belonging to some prior time
(used informally especially for emphasis)
of a very early stage in development
just preceding something else in time or order

Also: one-time, honest-to-god, honest-to-goodness, sure-enough.