Stir parsley, cilantro, shallot, garlic, oregano, red pepper flakes, vinegar, lemon juice, salt, and pepper in a bowl.
Whisk in olive oil until the mixture looks loose and glossy; taste and adjust seasoning.
Scoop out about one-third of the chimichurri and set it aside for serving.
Pat shrimp dry, then toss with the remaining chimichurri; marinate 10–15 minutes while you heat the pan.
Warm neutral oil in a large skillet over medium-high heat until shimmering.
Sear shrimp in a single layer, 1–2 minutes per side, until pink and just curled; cook in batches if needed.
Toss hot shrimp with the reserved chimichurri off heat and serve right away.