Question

Why 2nd Electron Gain Enthalpy of oxygen is positive?

Answer 1

The first electron affinity is negative because energy is released in the process of adding one electron to the neutral oxygen atom.  
Electron affinity can be thought of as a measure of the attrcation between the nucleus and the newly added electron, the higher the attraction the more negative the value, so if the value is positive then there must be repulsion not attraction. 
When the second electron is added you have to put energy in, rather than energy being released. For the first electron affinity we were adding an electron to a neutral atom, but now we are adding a negative electron to a negative ion so they will repel each other. So to overcome this electrostatic repulsion energy must be added in.

Answer 2

Answer:

The first electron gain is defined as enthalpy change

The second gain is termed as endothermic as it gains energy in a series of reactions due to repulsive forces rendering the second gain be positive