The Optio- Gallic "G" Weisenau Helmet Portrait - Original oil painting
An "Optio Legionis" was a position in the Roman army similar to that of an executive officer, acting as a deputy centurion. Chosen by his Centurion, his main function was as an "optio canturiae" - to assist in keeping troops in order by enforcing the orders of the Centurion, taking over command in battle should this need arise, and help with administrative duties.
The Optio's helmet would have been crested down the middle and flanked on each side by feathers. In this painting, the Optio is wearing an Imperial Gallic "G" helm, the Weisenau version.