Why does the effective nuclear charge for the 1s orbital increase by 0.99 in going from oxygen to fluorine but only increases by 0.65 for the 2p orbital?

This is due to addition of an extra proton on going from oxygen to fluorine as well the screening effect of the 1s orbital experienced by the 2p orbital.

Explanation:

When a proton (which is positively charged) is added to nucleus of the oxygen atom to form flourine, there is an increase in the effective nuclear charge and the 1s orbital feels the effect more.

Also, due to the screening effect of the 1s orbital, nuclear attraction is felt less on moving to the 2s orbital. Thus, the effective nuclear charge is lesser.