how dense is oak wood?

Answer 1

There are many species and heartwood is more dense than sapwood, but the range is 0.6 - 0.9 × 103 kg/m^3.
See below:

Answer 2

According to the 26th edition of Machinery's Handbook, page 377, the weight of white oak is 63 pounds per cubic foot in the green condition, but only 47 pounds per square foot when it had been air-dried. Red oak weighs 64 pounds per cubic foot green, and 44 pounds per cubic foot air-dried.

Of course, being a natural material, the density of oak will vary according to species, growing conditions, and how dry the wood is.