What is OBJ?
Wavefront OBJ is the closest thing 3D has to plain text: a list of vertices, one per line, then faces referencing them by index. It has been readable in every 3D tool since 1990, which is why it survives as the format people fall back on when nothing else will open.
What is OFF?
OFF (Object File Format) is about as small as a mesh format gets - a header, a count of vertices and faces, then the numbers. It turns up mostly in academic geometry work and mesh-processing datasets, where being trivial to parse matters more than being widely supported.