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OBO is when you're online, but your friends think you're dead. You're typing messages, but your profile says you're offline. It's like being in a group chat and pretending you don't exist.
I was OBO during the whole math test. I was online, but my teacher didn't know.
He was OBO for three days. I thought he died.
I was OBO while my friend was crying. He didn't know I was there.