Here's why:

* Electronegativity: Fluorine is the most electronegative element, meaning it has a strong pull on electrons. This causes its electrons to be pulled closer to the nucleus, resulting in a smaller atomic radius.

* Effective Nuclear Charge: Fluorine has a higher effective nuclear charge (the positive charge experienced by an electron) compared to oxygen. This stronger attraction between the nucleus and electrons pulls the electrons closer, shrinking the atomic radius.

* Electron Shells: Both oxygen and fluorine have their valence electrons in the second electron shell. However, fluorine has one more proton in its nucleus, which increases the attraction for electrons, leading to a smaller atomic radius.

In summary: Even though oxygen and fluorine are in the same period (row) of the periodic table, fluorine's higher electronegativity, effective nuclear charge, and one additional proton result in a smaller atomic radius compared to oxygen.