This mosaic of 22 Viking 1 Orbiter images (f473s13/17/19/21/23/25/27/29, f474s17/19, and f474s21 to 32), taken on July 11 and July 12, 1980, shows the complex caldera at the summit of Olympus Mons. South is at the top.
The caldera, a composite of as many as seven roughly circular collapse depressions, is 66 by 83 km across. The lowest parts of the floor are over 4 kilometers below the rim of the caldera.

(Image Credit: NASA/JPL/Arizona State University/Mosaic by astroarts.org)

High-resolution PNG (4475×4610 pixels; 6.38 MB)

Note: This is the most complete high-resolution Viking Orbiter image mosaic of the Olympus Mons caldera which has been published to date.
For comparison, here is a mosaic of the Olympus Mons caldera produced by A. Tayfun Öner in 1997 and published on Calvin J. Hamilton’s website “Views of the Solar System”, and here is a mosaic produced by JPL and published in NASA’s Planetary Photojournal.