well... i dunno what to suggest about the oil since i've done any myself, but i think you could go further with the colours. Add some different colours...like you usually do on digital stuff.
The construction lines are interesting to see. But i think the arm pitt areas are a could use a bit more attention =)...
The second and third pics are good...but strangely blurry. I'm sure you can add some nice solid planes and details.
And to finish, the pencil one is cool. Maybe smooth the lips a bit more like you did on the guy at the top left corner.