The Brief: "ONG" is an abbreviation for "On God." It is used when referring to strong belief in something.

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The abbreviation “ONG” stands for “On God.” It is used to indicate strong belief or trust in something, essentially swearing by it. It is an extension of the phrase, “I put that on God.”
“Ong” is commonly used on Twitter and TikTok.
https://twitter.com/mersaydez17/status/1293963455213514753?s=20