“Everything really is going to be okay. (And if not okay, then at least comic.)”
Source: Eat, Pray, Love
The Curious Case of David Fincher (2007)
“Everything really is going to be okay. (And if not okay, then at least comic.)”
Source: Eat, Pray, Love
                                        
                                        "The Perils of Being Pauline," interview with Francis Davis, The New Yorker (October 2001). 
Interviews
                                    
Interview for Vogue magazine, November 2008.
“Everything will be okay in the end. If it's not okay, it's not the end.”
                                        
                                        Also found with the alternative spelling: Everything is okay in the end.  If it's not okay, it's not the end
Found anonymously on Usenet in  2000 https://groups.google.com/forum/message/raw?msg=alt.support.divorce/gKiyfcAYreo/jjuc6KTu_NAJ.  First known attribution to Lennon is from  2011 https://groups.google.com/forum/message/raw?msg=stow-ma-apple-barn/45MNk9KiGsY/vaq6pr8hgI0J. 
Disputed 
Variant: Everything is OK in the end. If it's not OK, it's not the end.
                                    
“I found the secret to life; I'm okay when everything is not okay”
                                        
                                        "Upside Down". 
Songs
                                    
Cinematcal, April 4, 2007.