Although OSLO appears to the casual user as an ordinary windows application, it is actually a combination of an application and a macro language. Some of the commands supplied with the program are part of the core program, while others are written in the macro language. It is not normally observable to the user which is which. There are two macro languages in OSLO. OSLO Light contains SCP, an interpreted language. OSLO PRO and OSLO SIX also contain CCL, a compiled language. Both languages use C syntax.

Macro commands written in SCP are executed by preceding the command name with an asterisk, hence the name "Star command".

Advanced users can write their own star commands and integrate them with ones supplied by Sinclair Optics, because the menus supplied with OSLO can be changed by the user. Since these commands are often required only to support special needs, they do not need to have the scope that internal commands do, so it is very easy to add features to the program.