OSLO is fundamentally an interactive program. After each command, the requisite action is performed and the display is updated. This means, for example, that the surface data spreadsheet is always up to date. There are a few exceptions, mostly relating to entering optimization data, but for the most part OSLO is based on an interactive model. Resulting from this model are the concepts of a current wavelength, configuration, object point, and spot diagram. Once these quantities are set, they remain set until changed by the user, or by some command that resets them to a different value. This increases the speed and efficiency of the program. In the case of the spot diagram, for example, once a spot diagram has been computed, a wide range of analyses can be carried out without retracing the rays.

Prominent examples of the interactive organization of the program are the so-called interactive design windows used in OSLO. In the interactive design windows, lens parameters are attached to graphic sliders. Whenever a slider is changed by the user, the lens parameter changes, and OSLO updates a particular evaluation function (ray trace, spot diagram, etc.). The user senses that the program is responding in "real time".