Haskell Replace characters in string with string

This is an extension of this question: Haskell replace characters in string

I would like to tweak the following expression to replace a char with a string

let replaceO = map (\c -> if c=='O' then 'X'; else c)

In the end, I would the following results (XX can be a string of any length):

replaceO "HELLO WORLD"
"HELLXX WXXRLD"