Clothes, mainly.
When meeting with an Officer Selection Officer (OSO), it would not hurt to dress a little bit more professional–like candidates on liberty. It’s not mandatory, but you want to consistently make a good impression in every interaction with your OSO.
Obviously, when coming to the OSO’s office for a PFT or other workout, wear your personal, appropriate workout clothing.
Like everything else in the Marine Corps, this is not rocket science.
What Really Matters
Focus on the important components of the interview: the questions you will have for each other as the OSO interviews you and as you “interview the Marine Corps.”