The 23-year-old midfielder enjoyed a successful loan spell with the Championship outfit last season and their bid was accepted a month ago.
Holloway told his club's official website: "I am absolutely delighted to have signed Charlie. He played a massive part in keeping the club in the division.
"Having watched all the DVDs from last season and listened to the staff and supporters, I know exactly what he can bring to the club.
"I would like to thank (club president) Mr Belokon for making this transfer happen.
"He has put an unbelievable amount of trust and faith in me and I am extremely grateful. I am sure he will be delighted as all the supporters and I are at signing Charlie."