The 21-year-old, who impressed in Australia's World Cup campaign, is joining Bundesliga club FC Koln from Sydney FC.
Okon-Engstler's transfer is reportedly set at one million Euros ($A1.6m) for the Sky Blues, who signed him on a three-year deal in July last year.
"Obviously very emotional, very sad, that we weren't able to progress to the next round of the World Cup," he said.
"Just seeing some of the older players and how emotional they were, knowing that it's their last World Cup ... it really hit me, I think, after the game.
"It just makes me even more grateful to experience it ... it gives me so much motivation to go back to keep improving every day and try and play at the highest level every week."
Okon-Engstler was born in Belgium while his father, former Australian international and current Socceroos assistant coach Paul, was playing in the Belgian Pro League.
He moved to Australia in 2006 as a one-year-old when his father signed for the Newcastle Jets.
Okon-Engstler's move comes with 18-year-old teammate, defender Lucas Herrington, weighing offers from Europe's top flights.
Herrington, at the US with the Colorado Rapids, is believed to have piqued interest in powerhouses including Barcelona, Liverpool and Bayern Munich.
Herrington impressed at the World Cup where he was calm and composed until a late blemish, missing a penalty in the shootout as the Socceroos again fell at the knockout stage.
The Brisbane Roar product's value is estimated around $25m, with Barcelona's opening bid of around $8m turned down by Colorado.
And teammate Nestory Irankunda's stunning goal in Australia's cup opener lifted his price on the market amid keen interest.
Irankunda's future remains uncertain after his first cup offered flashes of brilliance.
The 20-year-old is at Watford, with the English Championship club rejecting a $16m bid from Premier League side Hull City.
Irankunda's value is thought to be double that amount, with other English clubs and some in Italy understood to be interested.
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