It is important for the chicken not to have any extra moisture, dry it with a few paper towels, and after that cut the chicken into 1-2 inch chunks.
Heat up your pressure cooker: press Sauté -> click on the Adjust button -> select More to get the Sauté More function, which means that the food will be sautéed over medium-high heat. Wait for the Instant Pot indicator to read HOT.
Add the oil to the hot Instant Pot, add the chicken and sauté for 2-3 minutes, stirring a few times. Cook until it just starts to get golden. When sautéing it, stir constantly so it doesn't stick to the bottom of the pan.
Also, after you sauté the chicken, check if bits stuck to the bottom, in that case, deglaze the pot with 1/4 cup orange juice and scrape them with a wooden spoon. If you leave the bits stuck to the bottom, they may burn or cause the pot not to come to pressure.
If you want a truly golden-brown chicken, brown it on the stovetop, as the Instant Pot isn't really good for that.
Add the sauce ingredients to the pot: remaining 3/4 cups of orange juice, minced garlic, ginger, soy sauce, white sugar, brown sugar, rice wine vinegar, orange zest, and Sriracha sauce. You can skip the Sriracha sauce or add more if you prefer your food on the spicier side.
Add the tomato sauce if you are using it. The tomato sauce adds a tanginess to the overall sweet recipe, and it makes it taste more savory. It is based on your own preference. I like it both ways: with and without the sauce. You can use less, or completely leave it out if you want a pure orange aroma.
Stir gently until all the ingredients are combined and coated in sauce.
Close lid, select Manual, and select 5 minutes on High Pressure. Make sure the vent is closed.
Use a 10-minute Natural Release. Turn off the heat. Release the remaining pressure by opening the vent. Open the lid.
Select again the Sauté function, on LOW. In a medium bowl combine 2 tablespoons of cornstarch with the orange juice, whisk until all combined with no lumps.
Add the mixture to the Instant Pot and gently stir to combine. Cook on the Sauté function for a few more minutes, stirring gently, until the sauce thickens. Simmer for 2-3 minutes.
If you want the sauce even thicker, mix one more tablespoon of cornstarch with orange juice and add it to the pot.
Let the Orange Chicken stand for 5-7 minutes, the sauce will thicken more.
Serve over rice and garnish with fresh chopped green onions and extra orange zest.