I am not familiar with Orcad Layout but I believe OrCAD PCB Editor is very similar to Allegro PCB Editor so everything I provided so far is how I have done it inside Allegro PCB Editor.
Its really up to you as far as where the symbol origin is placed. There is some Allegro SKILL Code on Sourcelink that will automatically generate the Body Center for you but I have never used it to give any other further guidance nor do I know if it will run inside of OrCAD PCB Editor.
As far as the Assembly shops placement file, it is best that the placement data does not need any modification or maybe just slight modifications. From past experiences with other companies, clean placement data will make the whole process go much faster and prevents any mistakes that may cost you extra time and money.
You have the ability to output generic placement data from the File menu (File > Export > Placement) and specify whether the data is driven by Symbol Origin, Body Center or Pin 1. Selecting Body Center will use the data provided on Package Geometry > Body_Center first and if it is not present it will calculate the Body Center using the Place_Bound_Top Shape.
Hope this helps,