The short answer is “Yes, but…” with the “but” being about how glossy the surface of the acrylics you’re using is, which has am impact on how well the oil paint sticks to it.
The longer answer is yes, oil paint can be used over acrylic that is definitely dry. With thicker layers, it must be dry all the way through, not only the surface.
Oil over acrylic is a useful approach for speeding up the drying time of a painting, though whether you ultimately call it an oil painting or mixed media is debatable. Using “good student” acrylic paint for a coloured ground and/or the initial underpainting also saves money.
The only possible issue I’ve heard about is an adhesion problem if the surface of the acrylic is very glossy and slick. This is because the bond between oil paint and acrylic is not a chemical one but rather a mechanical one (think “glued together” rather than “mixed”). There needs to be something for the oil paint to grip into, so matt acrylics are better than gloss. Or use some matt medium or thin the acrylic with water so it doesn’t fill the tooth of the canvas completely, giving the oil paint something to grip onto. Or rub a piece of sandpaper across the surface to roughen it.
An information leaflet published by Golden Artist’s Colors says: “While we have done studies of the glossiest of our acrylics under oil paint films and have not seen any signs of delamination, we want to err on the safe side and suggest the films should at least be matte finishes.”1
1. Priming: Acrylic Gesso Under Oil Paint, Golden Artist Colors. Accessed 31 July 2014.