Oxygen is the most abundant element in Earth's crust by mass. It makes up roughly 46% of the continental crust, ahead of silicon at about 28%, aluminium at 8%, and iron at 5.6%.
This abundance does not mean the crust contains much free oxygen gas. Most crustal oxygen is chemically bound in minerals, especially silicates and oxides. Silicon and oxygen combine to form silicate minerals, the major building materials of rocks such as feldspar, quartz, mica, and pyroxene.
The figures are estimates because Earth's crust is not chemically uniform. Continental and oceanic crust differ, and even continental crust varies by location and depth. The question's phrase “by mass” also matters: abundance by number of atoms would produce different comparisons because elements have different atomic masses.
A common mix-up is confusing the crust with the whole Earth. By mass, iron is the leading element in Earth overall because much of it is concentrated in the core, while oxygen remains the leading element in the crust.