Shout out
S/O is shorthand for "shout out," a message meant to send thanks, congratulations, or appreciation to a person, organization, etc. People often use S/O on social media, but they may also use it in texts and online messages.
For example, a person may post on a social site, "S/O to my boy Blue for graduating from university today!" Or, you might message your friends, "Thanks so much for coming out and supporting our fundraiser today. Also, special s/o to Ruby for making the delicious cupcakes!" S/O is similar to H/T, which people use on Twitter to acknowledge someone or someone's work.
Example
S/O to my mom on Mother's Day for all that she's done for my brothers and me
Happy Mother's Day, Kris! Love you!
Take the pop culture acronym challenge
Online auction and sales sites use the acronym S/O to mean "sold-out." For example, you may see S/O in eBay listings or on product pages in online stores.
Much less commonly, you may see other people use S/O to mean "sold-out" on forums or social media. When used this way, S/O may refer to a sold-out item or a person who "sold out," by betraying their principles.