Excel Conditional Formatting if one column equal to another

Excel Conditional Formatting if one column equal to another

This should be the simplest example of conditional formatting but I can't
figure it out. I havave two columns seperated by several dozen other
columns ('C'-original data, 'BO'- calculated value) and I need to
highlight the calculated value if it is not equal to the original value. I
can do this on a cell by cell basis but can't see how to apply it to a
whole column withoug adding a rule for every row (there are many 1000s of
rows).
What am I missing?