Why does oxygen have a higher electronegativity compared to chlorine?

The  electronegativity   of fluorine  compare  to the  electronegativity   of oxygen and chlorine as below

Explanation:

Fluorine  has the highest electronegativity than oxygen, and  chlorine.

 Explanation

Generally  the  electronegativity  increase   across the period ( from the left to the right)  and  decreases down  the group.

Both oxygen and fluorine  are in period 2   in  periodic   table. since  fluorine is on  fur  right side as compared to oxygen , fluorine has higher  electronegativity than  oxygen.

On the  other side  Fluorine  and  chlorine are  in the same group in periodic table. fluorine  is at  the top of  the group  while chlorine  is at  the bottom therefore fluorine has  higher electronegativity  than chlorine.

we can conclude that the melting point of table salt is above room temperature (it's actually 801 deg c). the melting point of mercury must be below room temperature (- 39 deg c).