As commenter pumpkinhead noted, the following formula is used to calculate cranial capacity in living people.

Source J.P. Rushton

This regression equation is over 100 years old so it may underestimate the crania of people today,  though if everyone in the modern developed world is underestimated to a similar degree, it might still be useful for measuring relative differences.

It’s very hard to compare the cranial capacity of two people just from looking at photos because as commenter pumpkinhead has noted, small differences in closeness to the camera can distort the relative sizes.  Nonetheless when two people are sitting behind the same desk,  you might get more reasonable results.

Putting a ruler on my computer screen, it seems that Oprah’s cranium is 1.205 as long as Ellen’s.

And 1.26 times as high.

To determine head breadth, we need a photo where they’re both facing the camera.

From here it seems Oprah’s head breadth is 1.017 times as great as Ellen’s.

The following are actual head dimensions from the U.S. military.

If we conservatively assume a woman as intelligent as Ellen has the head dimensions of just the average white woman (L: 186 B: 144 H: 124:) then Oprah should have the following dimensions: L: 224 B: 146 H: 156

This gives Oprah a cranial capacity of 1874 cc (1.48 times as big as the average white woman’s) using the Lee and Pearson formula, but given the huge error that can occur from trying to measure physical objects from  photographs,  I could be way off.  However such error would largely cancel out across the three different cranial dimensions measured, increasing the accuracy of the overall cranial capacity estimate.