Good ol' Proton
Electrons barely weigh anything so you can neglect that negative bad boy. Carbon has 6 protons and about 6 neutrons to yield an Atomic Mass Unit (AMU) of 12. Oxygen totally has the same reason behind Carbon, 8 protons and 8 neutrons.
Protons weigh the same amount as a neutron which is 1 AMU.
So 1 proton = 1 AMU
Hope that helps ya!