Is there a way to paint acrylics over oil underpaintings?
Fresh oil paints (less than 1 year old) are not going to have the proper adhesion for acrylics. Even after the oil films have properly cured, they are often too tight of a film and too hard for good adhesion with the acrylics. In theory, you could use our MSA Varnish as the intermittent layer, as it will adhere to the oil paint film, and the acrylics have satisfactory adhesion to it, but this is a reversible product and can lead to other archival and conservation issues.

What is the oil underpainting offering that the acrylics are not? If you need more open time for better blending or softer applications, you may be able to add Acrylic Glazing Liquid to the paints, or up to 30% Retarder to the paints to achieve better scumbling techniques. This approach would offer a more sound approach to the painting technique, as you are only using one medium, not combining them.

Is there a transparent primer with which I can paint over acrylic paintings so I can paint oils over the acrylic?
We commonly recommend the Matte Medium or the Fluid Matte Medium brushed evenly as a transparent gesso. This adds tooth to glossier acrylics and can also be used as the initial primer for times when the support needs to show. Preventing oil penetration into the substrate is best done prior to painting, and the GOLDEN GAC 100 works quite well as an oil barrier. Apply 2 coats of this product before priming. Another thought is we make a paint line called High Load Acrylics (HLA). This paint has the highest amount of pigment and is comprised of mostly inorganic pigments, such as umbers, ochres, and Cadmiums. It can be thought of as a colored gesso, as it is thinner than our Heavy Body (tube acrylics) like a gesso, and has excellent tooth and porosity for subsequent paint layers. Using the HLAs, one would not need to apply a transparent primer over them before the use of oils.

If I wish to use acrylic on unsized canvas and then wish to use oil paint later on, will the thin layers of Acrylics seal the canvas against the oil paint?
If you intend to use acrylics under oil paints, apply the stains directly to the canvas, then apply several coats of the isolation mixture, then a coat or two of Matte Medium. This should allow adhesion of the two paints and protect the canvas from the oil. To check for this, make a test panel, apply the acrylics as mentioned, then apply a few drops of linseed oil onto the canvas and watch it over the next few weeks. If the drop remains on the surface, then you have adequate protection from oil penetration.