Why do selenium , tellurium and polonium have more negative electron gain enthalpy than oxygen?
Electron Gain Enthalpy becomes less negative down the group.

This is true, as a general trend. But the 2nd period is an exception to this trend.
In p block, it is observed that the EGE of some 2nd period elements is actually less negative then their 3rd period counterparts.
In the case of oxygen, it’s EGE is even less than selenium, tellurium and polonium. It’s because oxygen has very small radius and thus have high electron density. Due to this, the incoming electron faces a lot of repulsion from the already present electrons.
So, it takes more energy to add an electron to an oxygen atom i.e. oxygen has less negative EGE. On the other hand, selenium and tellurium have much larger radius and thus the electron density isn’t very high. Hence, their EGE is more negative as its easier to add an electron to them.