[Scythels]

Could very well be the OSS, really it probably is.  With only spherical lenses and in a perfectly aligned system astigmatism is forbidden in the middle of the picture.  OSS introduces voluntary decenters.  For prime lens design they are relatively easy to design with because the beam of light is fixed.  In zoom design they are difficult to work with because the beam will "walk" between the different zoom positions so the effect of the asphere changes.  Add in OSS and you add a second layer of beam walk.  If the OSS group's motion is not perfectly constrained to one axis it's likely that the decenter interacts badly with the system and introduces astigmatism (which is what this looks like it is).