"OOTD" stands for outfit of the day. It is an acronym developed by young female fashion bloggers. Bloggers and other social media users use this hashtag on photos of their daily outfits.
The phrase is usually associated with the photography-sharing social media site Instagram, where it began to appear around late 2012 or early 2013. According to the Huffington Post, there are over 26 million photos tagged "ootd" on Instagram as of 2014. It has also spread to Pinterest, Tumblr and other social media sites. The short acronym has also become popular on Twitter due to the site's character limits.