If I am understanding correctly, Then I would do a Table lookup of OBX5 and store the result in a temporary variable @obx5 or something.
Then you could do if logic on the @obx5 and if the result is NOT @null — or your Table default value (if your using a default value)
Then you can map @obx5 to Obx3.
I hope this makes sense.
Ryan Spires