Why does oxygen atom allow positive charge on its surface when its electron pair is in resonance with benzene ring?

Oxygen atom is a highly electronegative element.

Why does oxygen atom allow positive charge on its surface when its electron pair is in resonance with benzene ring?

The reason can be that its octet is complete so oxygen is not disturbed by its positive charge.

• Oxygen having a positive formal charge is not all that uncommon: see $\ce{H3O+}$. – orthocresol Mar 5 '17 at 8:17
• @orthocresol but also it is not that stable as it dissociates and gives H+ ion , chlorine atom also bears positive charge in chloro benzene resonance (+M) and anisole also shows +M effect. – user7658311 Mar 5 '17 at 11:08