Can I paint over wood that has been treated with a wood preservative?

This depends on what sort of preservative and what sort of paint is being used. Normal tanalised wood for example can usually be painted with any suitable exterior wood paint. If the wood is to be treated with an exterior wood preservative, then painted with a water based exterior wood paint, it's important to ensure that the wood preservative being used does not contain any wax, oil or silicon, as these will repel the water content of the paint and prevent it from sticking or adhering to the wood. We recommend that when preserving and painting wood, a test area is treated first to check product suitability and compatibility.