Occitan and Catalan are more similar to each other than Catalan to Castillian or Occitan to French.
Edit: bambax said “Occitan is a lot like Spanish (unlike, say, Breton or Basque [...])”. He could have said “is a lot like Catalan”, or “is a lot like French”, or “is a lot like Gascon”. All are true to different degrees.
Occitan and Catalan are more similar to each other than Catalan to Castillian or Occitan to French.