El Mizouni has a year left on his contract at Ipswich Town and spent last season on loan in League One at Leyton Orient.
A number of clubs have been keen on landing the Paris-born midfielder since the transfer window opened this summer and Barnsley are amongst them.
The Oakwell outfit pushed hard to bring in El Mizouni, but they have been beaten by Oxford United.
The U’s are splashing £400,000 to take the midfielder to the Kassam Stadium and he will pen a three-year contract.
A product of Ipswich’s youth set-up, El Mizouni has had loan spells at Cambridge United and Grimsby Town.
He made 40 appearances in League One for Leyton Orient over the course of last season and was booked nine times.
El Mizouni played against both Barnsley and Oxford United, giving both clubs a good chance to take an up close look at him.
Now the 23-year-old will prepare to sink his teeth into Championship football.