I looked at operation drawings in the drawing register but cannot see how this works as when you go into the stocked part of works order the operation drawing tab is missing. There does not seem to be any method of how you link an operation drawing to a part.
In the drawing register there is an Operation Drawings tab, if the Drawing number and the image are added it is then possible to set an associated operation. To do this:
Go to the route then click F5 in the pic column.
Click Import from Drawing Register.