The OSL is a syntax for choosing Selectables. There are four generic types of Selectables: graphs, subgraphs, vertices, and edges. In a molecular model, these correspond to the entire molecule, the residues, atoms, and bonds. For a non-molecular model, the only relevant Selectable type, graph, corresponds to the entire model.
The OSL is almost exactly the same as the Midas atom specification syntax as implemented in the Chimera Midas emulator. A detailed description of that syntax can be found here. There are three features found in Midas atom specifiers that are not found in the OSL. They are: