Have a look at the last picture on this page
http://deepspaceplace.com/at12in.php
In this case the item labelled 'coma corrector' goes all the way through the full length of the focuser and ends in line with the OTA wall.
If you were to add an OAG, it would need to go in the space labelled "adaptor"
The filters and guide chip are inside the camera housing :-)
Does that make sense?
James