Oxygen has three stable isotopes, #99.76%##""^16"O"#, #0.04%##""^17"O"#, and #0.20%##""^18"O"#. Oxygen has the atomic number 8, which means the nuclei of its atoms have 8 protons. A neutral oxygen atom as also has 8 electrons.
The oxide anion has a charge of #2-#. This means that it has gained two electrons from another element, such as sodium or magnesium. All oxide anions, #"O"^(2-)"#, have 8 protons and 10 electrons. The number of neutrons depends on the isotope. Since you didn't specify which isotope, I'll give the answer for all three isotopes.
The mass number of each isotope is the sum of the protons and neutrons in the nucleus. It is also the number written as a superscript to the left of the isotopic symbol (e.g. #""^16"O")#. If we know the atomic number (8), we can subtract that from the mass number to determine the number of neutrons.
#"Mass number"-"atomic number"="number of neutrons"#
#""^16"O"^(2-)# has 8 protons, 8 neutrons, and 10 electrons.
#(16-8="8 neutrons")#
#""^17"O"^(2-)# has 8 protons, 9 neutrons, and 10 electrons.
#(17-8="9 neutrons")#
#""^18"O"^(2-)# has 8 protons, 10 neutrons, and 10 electrons.
#(18-8="10 neutrons")#
