Oil canning is the unstable movement of a broad panel between two shallow shapes. The surface may pop under light pressure, rock on a flat base, or appear wavy even when the perimeter dimensions are correct.
Large nominally flat areas have little geometric stiffness. Uneven wall thickness, differential cooling, residual forming stress, trim-induced release, and an unsupported perimeter can push the panel into an alternate shape.
Corrective action
The first step is to determine whether the instability exists on the tool, after demolding, or only after trimming. A panel that changes after trim is carrying locked-in stress. A panel unstable on the tool needs geometry or thickness support.
A slight crown, rib, bead, or controlled curvature is often more reliable than attempting a perfectly flat unsupported surface. Cooling must be balanced, and trimming fixtures should support the part without forcing it into a shape it cannot retain. Additional thickness may help, but geometry usually provides the larger improvement.