The ZWO 7nm OIII filter is centered around the Oxygen III band of 501nm. Oxygen III or double ionised oxygen occurs at primarily at 500.7nm and secondarily 495.9nm. The highest levels of OIII are usually found in diffuse and planetary nebulae. The OIII emission lines were first discovered in the 1860s in spectra of planetary nebulae. OIII wavelengths relate to the green-turquoise-cyan in colour on the spectrum whilst the narrowband width makes the background space very black.
When used in conjunction with the narrowband H-alpha and narrowband SII filters they make up part of the Hubble Palette, used by HST to faint gases in nebulae.