What you shouldn't do is to start a painting in oils, which dry slowly, and then paint on top with acrylics, which dry quickly. But provided the canvas has been primed to be suitable for both oil paints and acrylics, it's fine to start a painting with acrylics and then finish it in oils. But with the caution that the acrylic paint shouldn't be too glossy or thick.
Some canvas is primed for oil paint only, and you shouldn't use acrylic on these. Most modern primers (or gesso) is suitable for both. Some artists use acrylics to start a painting because they dry so much faster, then finish the painting in oils. Be sure the acrylics have dried completely (all the way through, not just touch dry on the surface) before you start with the oil paint. If in doubt, leave thin acrylic paint at least 24 hours.
Don't use the acrylic paint too thickly and smoothly as you don't want to create a smooth surface oils can't stick on. The bond between oil paint and acrylic is a mechanical one, not a chemical one (think "glued" or "stuck together" rather than "intertwined" or "mixed"). Thin glazes of acrylic on canvas will likely not fill the tooth of the canvas completely, giving the oil paint something to grip onto. Matte acrylics are preferable to gloss, because it's a less slick surface, more for the oil paint to grip onto.
If you're worried about the issue of the different flexibility of acrylics and oils once they've dried -- acrylics remain flexible, oil paint becomes less so the more it dries -- consider painting on a rigid support such as hardboard rather than a flexible one such as canvas.
Mark Gottsegen, author of The Painter's Handbook, says there has been "anecdotal reference to the failure of oil paints applied over acrylic... but no hard and consistent evidence from conservators. A lot of the failures of paintings in general can be traced to faulty artist techniques..."1
An information leaflet published by Golden Artist's Colors on priming 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."2
1. Mark Gottsegen, Acrylic Underpainting for Oils, AMIEN (Art Materials Information and Education Network). Accessed 25 August 2007.
2. Priming: Acrylic Gesso Under Oil Paint, Golden Artist Colors. Accessed 25 August 2007.