All measurement functions are available for Oblique content as for any other, see Measurement Tools.
Especially for Oblique content, there are some extra function provided that are disabled in other contexts. These are typical measurements where one starts with a point at ground level, then moves up vertically to the top of a building, and then continues measuring an area in a forced horizontal plane. The aim is of course just to measure the top area, and nothing else. The top image shows this perfectly.
Just as any other measurement, this one can be saved for later download. It is then shown in red when the active measurement function is cleared.
See also Download Measurements as Objects.