I am sure it's fine. It is undoubtedly primed with acrylic gesso; or rather, it is highly unlikely it was sized and primed in a traditional oil technique. In the old days, sizing (protecting the support--the painting surface--from the adverse effects of coming in contact with oils) and priming (painting the surface white for a brighter painting) were two separate steps. The canvas was stretched, sized with an animal glue, allowed to dry and then primed using thinned out (with turpentine) white pigment. If the painting was done on a board, gesso--a mixture of glue, chalk and white pigment--was used. Since the 1950's acrylic gesso has become popular. Acrylic gesso is a polymer-based liquid which sizes and primes simultaneously. It is appropriate for both oil and acrylic supports and, because it is both cheap and easy to use, is the most likely material used to prime your oil painting block.
If you like, you can paint in acrylics, let them dry, and then paint over them in oils. What you can never, ever do is paint in acrylics over oil paint.