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!
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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.