I am currently (2004) OP Platinum. An associate of mine is NW member.

The biggest difference we have compared against is that when I fly on NW using OP account, ALL segments flown on NW count for CO OP, regardless of fare class paid.

When my associate flies NW using his WP number, he only gets credit for certain segments depending on the class of the fare....thus he gets gold for 2004 and I get platinum.

Of course if you get the miles, this isn't applicable...but usually I go be segments for my account status.