He has scored 10 goals already this season and was invited to train with the rest of the Harry Redknapp's squad last month.
Kozak said: "It was a great experience. I can also say that it has been partly a dream come true for me but it had some disadvantages as well.
"I could not spend the winter preparing with my team-mates and missed the fitness drills so I had to gain the fitness afterwards."
The 18-year-old has ambitions to play for a big club but is not planning to move on just yet.
"I am not thinking about leaving," he said.
"When I feel I am good enough to go to a better club, I will sit down with Mr Hadamczik (Opava's general manager) and we will discuss it."