OIFITS is a standard for exchanging calibrated, time-averaged data from astronomical optical interferometers, based on the FITS Standard (OIFITS is a registered FITS convention). OIFITS may be used to combine data from multiple interferometer arrays for joint analysis and/or image reconstruction. OIFITS-format files can be prepared from the raw interferometer data without using information about the detailed structure of the target object (i.e. without doing any astrophysical interpretation), yet once the data is in the format, it can be analysed without knowing the details of the instrument. Calibrated data from different interferometers can be treated in the same way (provided there are no residual systematic errors).