Optical coherence tomography (OCT) can be an important diagnostic tool in optics R&D and manufacturing. We illustrate this with four applications. (1) Imaging of vibration modes of MEMS membranes. (2) Strain mapping of VCSEL chips. (3) Optical frequency domain reflectometry (OFDR) of laser cavities. (4) Combined responsivity and 3D mapping of photodiode packages.