Can US pilots fly VFR OTT and/or VFR at night in Canada without additional ratings?
It looks like if you have a foreign instrument rating you automatically have VFR OTT privileges, but for night VFR you still need a Canadian night rating. 

CAR 421.44(7) for VFR OTT:


  An applicant who holds or has held an instrument rating issued by a
  Contracting State shall be considered to have met the Knowledge,
  Experience and Skill requirements specified above


As for night VFR, 421.42 doesn't have any equivalent section that gives credit for foreign qualifications.