I'm trying to specify a query in my model

$this->db
        ->select('*')
        ->from('library')
        ->where('library.rating >=', $form['slider'])
        ->where('library.votes >=', '1000')
        ->where('library.language !=', 'German')
        ->where('library.available_until >=', date("Y-m-d H:i:s"))
        ->or_where('library.available_until =', "00-00-00 00:00:00")
        ->where('library.release_year >=', $year_start)
        ->where('library.release_year <=', $year_end)
        ->join('rating_repo', 'library.id = rating_repo.id')

So, the trouble i'm having is with my or_where. I want the or to be restricted to only the available_until field. Currently, however, i'm getting results which have a language of German which isn't what i want. How do i restrict my or_where filter to the available_until field only?