IFF_OACTIVE can actually be set in either
ifp->if_flags or ifp->if_flags_tx - different
fields.

The driver actually doesn't set IFF_OACTIVE
in ifp->if_flags, although it may drop it
during shutdown.

The use of IFF_OACTIVE in ifp->if_flags_tx
is a bit more obscure.  The BSD driver for
the i82544 driver sets it when it's transmit
descriptor ring is full - see sean's comment
above about pending tx interrupt.  I copied
that behaviour in a recent commit to the
devnp-i82544.so driver.

--
aboyd