1. What does it mean?
OOTD is the English initialism for outfit of the day, used in Korean fashion captions and hashtags as well. It typically introduces an outfit photo, mirror selfie, or a short list of clothing pieces worn together.
Also written or said as 오오티디
2. The nuance
It does not mean the outfit is sponsored, newly purchased, or especially formal. Korean speakers may write 오오티디 when spelling the letters out in Hangul.
3. Examples
“오늘 OOTD는 셔츠랑 검정 바지로 끝.”
Today's outfit is just a shirt and black trousers.
“OOTD 사진 아래에 신발 정보를 적어 뒀어.”
I put the shoe details under my outfit-of-the-day photo.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.