Yes, open is playable
...and is worth 8 points.
Found in the following dictionaries
Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)
"Open" Definition
noun
A tournament or contest in which both professional and amateur players may participate.
An undisguised or unconcealed state: brought the problem out into the open.
An unobstructed area of land or water.
The outdoors: camping in the open.
open and unobstructed space as.
adjective
Accessible to all; unrestricted as to participants: an open competition.
Affording unobstructed entrance and exit; not shut or closed.
Affording unobstructed passage or view: open waters; the open countryside.
Available for use: an open account; the only course open to us.
Available; obtainable: The job is still open.
verb-transitive
Games To begin (the action in a game of cards) by making the first bid, placing the first bet, or playing the first lead.
Law To recall (an order or judgment) for a reexamination of its merits.
To break the continuity of; make a gap in: open a circuit.
To commence the operation of: open a new business.
To form spaces or gaps between: soldiers opening ranks.
verb-intransitive
Games To make a bid, bet, or lead in starting a game of cards.
To be performed, shown, or made available to the public for the first time: The play opens next week.
To be priced or listed at a specified amount when trading begins: Shares opened high and fell sharply.
To become open: The door opened slowly.
To become receptive or understanding.
idiom
Open fire To begin firing on.
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