why is oxidation no. of oxygen in OF₂ n O₂F₂ +ive?

Fluorine predominantly exhibits -1 oxidation state in almost all it  compounds. In both OF₂ and O₂F₂, its oxidation number is -1. Using this, we will calculate the oxidation state of oxygen in both the compounds. The overall charge in both these compounds is zero. So when we calculate the charge on oxygen in OF₂and O₂F₂, it comes out to be +2 and +1 respectively. You can also calculate it as follows:

OF₂

Let the oxidation number of oxygen be z. As there is no overall charge on the molecule, therefore we have

 z + 2 (-1) = 0

This gives z = 2

Thus the charge on oxygen in OF₂ is +2. 

O₂F₂

Let the oxidation number of oxygen be z. As there is no overall charge on the molecule, therefore we have

 2z + 2 (-1) = 0

This gives 2z = 2

or z = 1