* Atomic Radius: Atomic radius refers to the size of an atom. Boron has a smaller atomic radius than oxygen.

* Number of Electron Shells: Boron has 2 electron shells, while oxygen has 2 electron shells. The more electron shells an atom has, the larger it is.

* Nuclear Charge: Boron has a lower nuclear charge (number of protons) than oxygen. This means the electrons in boron are less attracted to the nucleus, leading to a larger atomic radius.

* Electronegativity: Oxygen is more electronegative than boron. Electronegativity is the ability of an atom to attract electrons. Oxygen's higher electronegativity means its electrons are pulled closer to the nucleus, resulting in a smaller atomic radius.

In summary: Despite having the same number of electron shells, oxygen's higher nuclear charge and electronegativity make it smaller than boron.