Using OR in conditional fields
just an observation from looking at the module documentation:

there is a note under interaction with other dependencies on the Edit dependency page which says this:

When this dependent has more than one dependee, how should this condition be evaluated against the others?
Note that sets will be grouped this way: ANDs AND ORs AND XORs

with a selection for AND, OR and XOR

This would indicate that you


define your dependency of field C on A,
define your dependency of field C on B
and select OR as your interaction with other dependencies