Now I have two different answers! One says put voltage on Two different slider shoes & One says Slider Shoe and Truck Ground! Now I am really confused !!
You did say MTH Operating Car. I was not aware that MTH made operating cars that required an OTC. I don't believe MTH even sells OTC contactors; one would think they would sell what it takes to operate their cars. I believe Lionel cars that required an OTC came with.
I assumed you might have been using "OTC" inter-changeably with UCS or MTH "Operating Track Section" as a means to activate an operating car via 4th/5th rail sliding shoes. Perhaps a bad assumption on my part.
Take a look at the manual of your MTH operating car. If the instructions reference the MTH "Operating Track Section" then apply voltage to the two slider shoes.