Remember we just told you that the slope was very gentle? Well, here's the side effect of that. For a gentle slope to turn into the highest known point in the universe, it has to be wide. Very wide. Wider, in fact, than your eyes can take in.
Olympus Mons is sixteen miles high at its peak, but it's also 374 miles in diameter. To give you a comparison, that's more or less the same size of the entire state of Arizona. Because of that, if you stood on the edge of the caldera - a sort of crater left by the collapse of a magma chamber - everything around you for as far as your eyes can see would still be the mountain, and the base of the volcano would be over that visible horizon.