I believe there may be a slight confusion in your question. Oxygen is actually more electronegative than chlorine, not more electropositive. Electronegativity is a measure of an atom's tendency to attract electrons in a chemical bond, and oxygen is generally considered to be more electronegative than chlorine.
Regarding the comparison of their first ionization energies, it is important to note that ionization energy is the energy required to remove the outermost electron from an atom in the gaseous state. Oxygen and chlorine both belong to Group 16 and Group 17 of the periodic table, respectively, and they have different electron configurations.
Oxygen has an electron configuration of 1s^2 2s^2 2p^4, while chlorine has an electron configuration of 1s^2 2s^2 2p^6 3s^2 3p^5. In both cases, the outermost electrons are located in the p orbital. Oxygen has six valence electrons in total, while chlorine has seven valence electrons.
The first ionization energy depends on several factors, including the effective nuclear charge (the net positive charge experienced by the outermost electrons) and the electron-electron repulsion within the electron cloud.
In the case of oxygen, it has a lower first ionization energy compared to chlorine for a couple of reasons. Firstly, oxygen has a smaller atomic radius than chlorine. As you move across a period from left to right, the atomic radius generally decreases due to increased effective nuclear charge pulling the electrons closer to the nucleus. The smaller atomic size of oxygen means that its outermost electron is held more tightly compared to the outermost electron of chlorine, which is further from the nucleus.
Secondly, oxygen has a half-filled p orbital (2p^4), which results in greater stability compared to the partially filled p orbital of chlorine (3p^5). Half-filled or fully filled orbitals tend to have lower energies and greater stability, making it slightly easier to remove an electron from oxygen compared to chlorine.
Therefore, despite being more electronegative, oxygen has a lower first ionization energy than chlorine due to its smaller atomic radius and the greater stability of its half-filled p orbital.