What is likely to happen to an atom of oxygen that noes not contain the maximum number of electrons in its outermost energy level?

An isolated oxygen atom has exactly eight electrons. If it had any other number, it would be an oxygen ion. But most oxygen atoms are not isolated -- they are chemically bonded to other atoms in molecules like water H2O or oxygen gas O2, or in networks like silica SiO2 An oxygen atom can be thought of as having three energy levels for its electrons. There is a 1s energy level for electrons bound very tightly and very close to the nucleus. It can accommodate 2 electrons. Similarly there is a 2s energy level that can accommodate 2 more of the oxygen electrons, in a less tightly bound situation, and a 2p energy level that can accommodate 6 electrons, but only actually has 4 electrons to accommodate in an isolated oxygen atom. An isolated oxygen atom is very greedy to fill those extra two electron slots. It cannot actually steal electrons in other atoms in most circumstances, because that would make a negative ion, where the extra electrons would not be at all tightly bound. What it can do is to steal other atoms to form new chemical bonds and make new molecules or networks. In these new arrangements the empty slots in the 2p energy level are filled by electrons shared in the new chemical bond To take a particular example, oxygen forms very strong bonds with hydrogen atoms, so much so that it can steal a hydrogen atom from a molecule of the generally unreactive gas methane, breaking a slightly weaker chemical bond as it does so: O + CH4 --> OH + CH3 OH + CH4 --> H2O + CH3 In this way, the isolated oxygen atom with 4 electrons in its 6 available 2p slots becomes a water molecule with all 6 of the 2p slots filled. These reactions where oxygen is able to steal hydrogen atoms that were originally bonded to carbon are an important part of the complicated series of reactions involved in burning fuel. That is the long answer. The short answer is that it is likely to undergo chemical reactions so that it can fill any empty slots with shared electrons.