The Oberth really doesn't work as a TOS ship. However, if you are insistent on trying it out, make sure the Nacelles are above the saucer. There has to be a way for the crew to get to the turbolifts in the Pylons, in your current version the Nacelles are in contact with the Saucer, leaving no place for the Lift shafts to get to the Pylons, and into the engineering deck.