There is no instances of the OHO beyond the one granted in the DLC, found in the Shrine of Resurrection.
That said, there are glitches that allow you to keep the OHO after the end of the trial.
Mini Game Storage: Keep the One Hit Obliterator
Effect: Lets you keep the One Hit Obliterator outside the quest it's meant to be used in
How To: Activate mini game storage with the Test of Wood escape glitch mentioned above, then start the Obliterator trial. You'll immediately fail the mini game due to having unequipped the Forest Dweller's Sword in favour of the One Hit Obliterator. Now, choose to restart the mini game. Monk Maz Koshia will say you've left the Great Plateau and that the trial has ended, but he won't take the Obliterator away. Instead, he'll take your current weapon, which is now the Forest Dweller's Sword. You'll then fail the Test of Wood again, and still have the One Hit Obliterator in your inventory, letting you use it whenever you want or hang it up in your house.
Notes: The One Hit Obliterator only does one damage outside of the Great Plateau, but will reduce Link to a quarter of a heart when used regardless. Dying with it equipped will not cause an instant game over either, and both Fairies and Mipha's Grace will now act as expected when using it.
As for the Object Map, this is a tool built from data mining the game's code - outside of this tool, there is no records anywhere of a "second instance" of the OHO - likely making it a bug in the tool's code.