https://en.wikipedia.org/wiki/Greek_alphabet#Glyph_variants
The letter omega (Ω) has at least three stylistic variants of its capital form. The "open omega", resembling an open partial circle with the opening downward and the ends curled outward, is the standard. The two other stylistic variants, seen more often in modern typography, resemble an underscored full circle, where the underscore may or may not be touching the circle on a tangent (in the former case it resembles a superscript omicron similar to that found in the numero sign or masculine ordinal indicator; in the latter, it closely resembles some forms of the Latin letter Q). The open omega is always used in symbolic settings and is encoded in Letterlike Symbols (U+2126) as a separate code point for backward compatibility.