Onomah has played for every England age group from Under-16 upwards and was a member of the squad that won the Under-20 World Cup recently.
He played 12 times for Spurs last season, including a Champions League appearance as a substitute against Bayer Leverkusen.
Villa were relegated from the Premier League in 2016 and finished a disappointing 13th in the Championship last season.
(Reporting by Steve Tongue; Editing by Toby Davis)
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