How to find out oxidation state when compound has multiple electronegativities
You can assign oxidation states with a method similar to assigning formal charges.


Draw the Lewis structure.
Assign each atom all of its lone pair electrons.
For bonding electrons assign all of the electrons to the more electronegative element.
For each atom perform the following calculation:
number of electrons in the atom in its elemental state - assigned electrons = oxidation state.


In your example, the oxidation number of each $\ce{H}$ is +1 and of $\ce{F}$ is -1.  The oxidation number of $\ce{C}$ is -2 and of $\ce{O}$ is 0.