How do I properly write a character who is an Officer?

You should change the character of your story. The best writing always comes from experience and observation.

Figure out what you don't know, create questions, then interview Officers to get those answers. Interview Cadets, too. Ensure that you are interviewing Infantry Officers since that is the type of character you have.

Either that, or change the character to an area in which you do know about.

SPC Austin Wilson couple things.

- As SFC (Join to see) said, you may want to write what you know.
- as MSG (Join to see) said you could try constructing a structured interview for an officer/Cadet willing to help you. What you have here is the "best self perspective" where if you are writing a flawed character, it may be kind of difficult.
- create a composite of two-three officers you have known that are close to your character. The risk here is if it is too close in personal detail and it is published, then you have people coming to you for various reasons.

Not sure how you plan to have this character unfold.

Edit: you may want to read Starship Troopers as it gives an interesting take on enlisted to officer and includes Officer Candidates leading troops in a Sci Fi setting. If you simply watch the film, I will lose my mind. The film and book are only tangentially related.

You may also want to try reading some of the books by David Drake, particularly those in the Hammer's series. My gut tells me they are composites of officers he knew while in Vietnam in 11th ACR.

You may also want to try reading Company Commander by McDonald. Harold Coyle also writes the subject well, as he was a commissioned officer, armor. The book series that starts with either Sword Point or The Ten Thousand. Ed Ruggerio 38 North Yankee. Ed Ruggerio was a an Airborne Infantry Officer. His characters were pretty realistic. Plus reading good writing helps with good writing.