I think oil and acrylic are too similar to be considered mixed media.
If it were water colour and oil, used in distinctively different ways on a piece, then sure.
True mixed media (IMO) would be completely different media, e.g. colour pencil with a collage or marker pen and poster paint.
Collages are usally mixed media, often involving the use of paint as-well as paper cuttings or photos.
At least I think so.