The 6.4 release of OSLO, from Lambda Research, features more than 40 additions and improvements. Among the highlights is the added capability to place multiple Interferogram (INT) files on any optical model surface. Interferogram files can represent a different source and type of error, e.g. freeform wavefront or surface deviations. Placing multiple interferogram files on a surface allows OSLO 6.4 to perform fairly complex tolerance modeling to predict the as-built performance of real optical systems. The attached diagram represents typical errors that might be modelled with separate interferogram files: (A) ‘print-through’ errors of a lightweighted mirror, (B) atmospheric turbulence, and (C) the composite error that would be found in the as-built system when interferograms A and B are combined. OSLO’s implementation of separate interferogram files allows users to adjust the size and shape of individual errors in an easy and efficient way.