Combine garlic, cumin, pepper, oregano, orange juice, lime juice, and olive oilin a large bowl and whisk. Season to taste generously with salt.
Transfer half of mojo to a sealed container and reserve in the refrigerator.
Add pork to remaining mojo and turn to coat. If desired, transfer pork and marinade to a gallon-sized zipper-lock bag and refrigerate 2 hours or up to overnight before continuing
For the White Rice
In a medium saucepan over medium heat, bring water to a boil.
Add rice, butter, and a large pinch of salt.
Bring pan back to a simmer then lower heat and cook, covered, 18 minutes, or until rice is tender and water is absorbed.
Remove from heat and let sit, covered, 5 minutes, then fluff with a fork and serve.