Electronegativity is the tendency of an atom to attract electrons towards itself in a chemical bond. Electronegativity generally increases as you move across a period and decreases as you move down a group in the periodic table.

Oxygen is located in the second period and group 16, while Selenium is located in the fourth period and group 16. Therefore, oxygen is smaller and has a greater nuclear charge, making it more electronegative than Selenium.

Here's a simplified explanation:

* Oxygen is smaller: Its electrons are closer to the nucleus, leading to a stronger attraction.

* Oxygen has a higher effective nuclear charge: The number of protons in the nucleus is greater than that of Selenium, leading to a stronger pull on the electrons.

Therefore, oxygen is more electronegative than selenium.