What is the mass number of oxygen?

That depends on the isotope of oxygen.
##(“A”)## is the sum of the ##”(Z”)##, which is the number of protons, and neutrons ##(“N”)## in the nucleus of a specific isotope of an element ##(“A=Z+N”)##. are named for their mass numbers. Oxygen has three naturally occurring isotopes.
The most common isotope of oxygen has 8 neutrons. Its atomic number is 8.
##”A=Z+N”##
##”A=8+8=16″##
This isotope of oxygen is oxygen-16.
Another isotope of oxygen has 9 neutrons.
##”A=8+9=17″##
This isotope is oxygen-17.
A third oxygen isotope has 10 neutrons.
##”A=8+10=18″##
This oxygen isotope is oxygen-18.
The following is the isotopic or nuclear notation of an isotope.