OLIcal calibrates OLI images to ETM+ images through modeling of ETM+ spectral indices from 6 of the 7 OLI reflectance bands. The process is nearly identical to the MSScal procedure, so please visit the link for details. The only difference between OLIcal and MSScal is that there are no coincident images between OLI and ETM+ so near date images are used to develop the linear relationship between the two sensors instead of coincident dates.