O_cwrite(): rewrote for clarity, replacing all the (Oobject *)self

O_cwrite(): rewrote for clarity, replacing all the (Oobject *)self
casts with a variable oself that has the proper type.  A smart
compiler may put this thing into a register.

(I'm not sure what good this does except satisfy my desire to
understand this function; I got a report about an uninitialized read
from Insure++ about this function and it hurt my eyes to even look at
it.  I gotta run away or I'll get tempted to reformat the entire
file...)