Per the title, an officer candidate is one who is in training for the officer corps but not yet graduation from the program.
This is the only regular episode that shows an NCO of a higher rank than Carter. He is a First Sergeant. That's 3 chevrons, 3 rockers, with a diamond replacing the crossed rifles in the middle. That's one pay grade higher than Carter. He is travelling with the captain on the inspection tour early in the episode. His stripes are hard to see as they appear very briefly as the captain enters the supply hut. But he is definitely identifiable as a First Sergeant. There is also a Master Sergeant sitting behind Carter and Gomer when they are talking the officer qualifying exam That's 3 chevrons and 3 rockers with crossed rifles. One pay grade above Carter.