N'Guessan, who is from the Netherlands, will sit out the 2018-19 season due to NCAA transfer rules. He has two years of eligibility remaining.
ETSU coach Steve Forbes said in a statement that "having the opportunity this upcoming year to sit out and work on his game and in the weight room will be invaluable to his future and ours."
N'Guessan played 26 games and made seven starts for Oklahoma State last season. He averaged 1.4 points, 1.9 rebounds and 9.4 minutes.
He started a total of 17 games in his two seasons with Oklahoma State. N'Guessan is fluent in four languages.