# What does O- mean in a NIFTI file name of an MRI?

I am exploring brain MRI scans. Using MRIConvert, I converted dcm files to NIFTI. Some of the outputted NIFTI files have O- in their filenames and I could not figure out what does it mean.

Example filenames:

xxxxxx_006_O-Ax_T1_FSE.nii
xxxxxx_007_O-COR_T2_FSE.nii