I need a 1 1/2" thick form for my TD longbow. Any reason I couldn't use oak boards that are 3/4" actual thickness and laminate them together? This will be an air hose form.

I have always been told to be sure and use dimensional lumber for molds and strong backs as they decrease the likelihood of the mold from warping.

Think of a wet board as it dries, it warps because the fibers shrink as moisture is evaporated.

Dimensional lumber has multiple layers of wood with the fibers running in multiple angles to the surface of the other layers. As they shrink the dimensional lumber pulls against itself and thus is less likely to warp.

I hadn't considered the warping when it goes into the oven either on the boards so I am going to stick with the plywood.

Having been a builder we always considered 2'x's as dimensional lumber, nominal as it is.
The way you could use the oak is if it were dry, thin, using mutiple layers, like plywood but not necessary as thin as those plys.  Probably be easier to use plywood, though.