Add the oyster sauce, soy sauce, rice vinegar, Sriracha sauce, ginger, garlic, and ground black pepper into your Instant Pot.
Spread the onion in the pot. Add chicken without overlapping. Close the lid and seal the valve. Press the “manual” button, set the pressure to "high", and set the timer to 10 minutes.
When the cooking is done and you hear the beep, immediately use a pair of tongs to turn the valve to release the pressure. Be careful not to put your hands or face near the valve, so the steam won’t burn you. The release will be quick, less than 1 minute.
Once the pressure has released, you will hear a small click. Open the lid and let cool for a few minutes. Use two forks to shred the chicken.
Add the water chestnuts. Turn on “Saute” function. Cook and stir for 3 to 5 minutes, until the water chestnuts turn a bit tender and the sauce is slightly thickened.
Add the green onion, if using, and toss again.
To serve, add 1 to 2 spoonfuls chicken with the sauce onto a lettuce leaf. Wrap it like a taco and enjoy!