Ohr can be translated as "light" and Shefa can be translated as "flow". They are often used interchangeably as we often say phrases such as "I saw the light" meaning "I have understanding" or "going with the flow" as in following an intuitive path. To study it further you might want to look at the numerical value of each word. Ohr has the same value as Raz - mysteries, so it is sometimes used more often to describe the mysteries or "light" that the Kabbala can shed for the student. Shefa on the other hand has a value of 110 which is the same as Chokmah - wisdom. This is a condensed version of a more complex definition but hopefully it sheds some Ohr.