Hi Jeremy,
I'm in the middle of changing over cameras on my scope at the moment, but what I was using until recently that worked OK with my RC8 was the AstroTech field flattener
https://www.astronomics.com/astro-te...rs_p17393.aspx
Not sure if it'd work with the RC6 but may be worth considering,
Without it, the stars in the corners of my QHY8 were a bit egg-shaped. Not tried the OAG without it, but I'd imagine it'd be like you describe. With it, the stars are not perfect, but ok for guiding.
With the RC6, the smaller focal length will make your radius of curvature smaller so the effect might be even worse for you.
Did you have some sort of flattener in the other scope you use?
Paul