Here are a couple of paintings that I've done over the past couple of days. Both utilized some Acrylic underpainting, but are essentially "Oil paintings."
The sunflowers were started yesterday and finished today. I had added Alkyd medium to the Oils to speed the drying, so that it was virtually dry over-night. That made it easy to glaze and brighten the final highlights this morning.
The barn was tackled in the same way; Acrylics underneath with Oils over the top. The difference is that I was able to get the amount of detail I desired without having to wait for it to dry, so it was all done in one sitting.
I use this approach quite often, (more and more, it seems) and it is one of my favorite ways to work. It eliminates a lot of the drying time associated with Oils and literally cuts the "working time" in half.
Acrylics, Alkyds and Oils,........ is that mixed-media? And does that matter? Not to me. It's still predominantly 90+% Oils, so I still call it an Oil painting.
All I know is that it works great for my style and work pace.
Every artist will eventually stumble upon the medium, approach, style and techniques which work for THEM. Don't be afraid to experiment until you find the right combination for YOU!