Combining Inverted Double Arch Below

Put it immediately after the letter it modifies. In phonetic writing, add the labialization mark after a base letter, as in k̫, to show that the lips round while the consonant is made. That extra lip movement is called labialization. The mark has no spacing width of its own, so it attaches to the preceding letter.

If you enter it before the base or leave it alone, a font may show it floating or put a dotted circle before it; the circle is only a display aid. Fonts handle combining marks differently, so alignment can vary even when the character sequence is correct.

What does omega below mean?

It marks extra lip rounding on the preceding sound. It modifies the sound represented by the preceding letter; it is not an independent omega letter.

The 1928 IPA chart is the earliest chart record in the historical archive and lists this mark for labialization. A 2021 Unicode proposal describes the older IPA form as retired, yet useful for distinguishing lip rounding made alongside a consonant from a rounded release (rounding added as the consonant ends) or labio-velarization (lip rounding combined with a back-of-the-tongue gesture), and for reproducing older phonetic sources.