OTSRs, and Horse collars are the same to me, they both go over my shoulders, but there is a slight deifference, which has to do with viewing.
However the dicitonary on Coasterglobe says this,
Horsecollar - A type of restraint that swivels down and over the shoulders. Not to be confused with an OTSR.
OTSR - Acronym for Over The Shoulder Restraint. A type of restraint that comes vertically down over riders' shoulders. These allow for a better view than horsecollars.
Call them wnat you want, but as far as im concerned teh 2 terms are interchangable.