He joins Hornet's boss Gianluca Vialli who has been an admirer of the Aussie captain for a number of years.
Since joining Boro from Forentina, back in the summer of 2000, Okon played only 28 games for Boro, in which, he failed to score.
Also on the transfer front, Steve McClaren admitted today that Boro had made a tentative enquiry for Dwight Yorke.
Speaking on Sky Sports he said "I know Dwight very well and the link's there from the days at Manchester United and with his availability we're bound to be linked with it."
"I know what a great player he is and what a great acquisition he would probably be for Middlesbrough. At the moment it's just a tentative enquiry that's gone no further.''
At £7m he would stretch Boro's finances to the limit, so I would expect Boro to hang on and monitor the situation as the season progresses, perhaps waiting until the close season before making any noises on the transfer market.