You can go from MNI space to orig space in the same way as one would go from TLRC space to orig space. Look at this previous link for "untalairaching" for tips.
[
afni.nimh.nih.gov]
<[
afni.nimh.nih.gov];
The "+tlrc" and "+orig" are the "view" part of the name. The "+tlrc" doesn't necessarily have to be in a Talairach space though. You can think of it as just in a standardized space, if you like. The AFNI GUI allows you to switch between the views either way. To change the view of a dataset, use 3drefit.
3drefit -view tlrc DSET+orig