optical emission spectroscopy 

Initialism: OES
https://doi.org/10.1351/goldbook.08513
Measurement principle of atomic emission spectroscopy in which the emitted electromagnetic radiation is in the range of frequencies from the infrared, through visible, to the ultra-violet region.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 726 (https://doi.org/10.1515/pac-2019-0203)