What the symbol for the oxygen isotope with 10 neutrons?

To write the symbol, we need to things, the atomic number and atomic mass.

if you go to the periodic table, you see that Oxygen's atomic number is 8.

for the atomic mass, we need to determine it. remember that atomic mass is equal to neutrons plus protons. Also, remember that the atomic number represent the number of protons, so

atomic number= number of protons= 8
number of neutrons= 10 (given by the question)

atomic mass= 10 + 8= 18

now we can write the symbol, I have drawn it and attach it.

remember than when you draw it, the mass goes on top, atomic number goes bottom.