The glyph is not a composition. Its width in East Asian texts is determined by its context. It can be displayed wide or narrow. In bidirectional text it is written from left to right. When changing direction it is not mirrored. The word that U+03BF forms with similar adjacent characters prevents a line break inside it. The glyph can be confused with one other glyph.
The Wikipedia has the following information about this codepoint:
Omicron (US: , UK: ; uppercase Ο, lowercase ο, Greek: όμικρον) is the fifteenth letter of the Greek alphabet. This letter is derived from the Phoenician letter ayin: . In classical Greek, omicron represented the close-mid back rounded vowel IPA:[o] in contrast to omega which represented the open-mid back rounded vowel IPA:[ɔː] and the digraph ου which represented the long close-mid back rounded vowel IPA:[oː]. In modern Greek, both omicron and omega represent the mid back rounded vowel IPA:[o̞] or IPA:[ɔ̝]. Letters that arose from omicron include Roman O and Cyrillic O. The word literally means "little O" (o mikron) as opposed to "great O" (ō mega). In the system of Greek numerals, omicron has a value of 70.