Heat 3 tablespoons olive oil in a large skillet over medium-high heat; cut zucchini into half-moons and cook until lightly golden on both sides, about 6 minutes total, then push to the sides of the pan.
Add minced garlic and cherry tomatoes to the center of the pan, season with oregano and salt, and cook 2 minutes until tomatoes start to burst.
Pat halibut dry, season both sides with salt, then nestle into the pan around vegetables and drizzle with remaining 2 tablespoons oil.
Cook halibut 5–6 minutes per side, depending on thickness, until it flakes easily with a fork.
Squeeze fresh lemon juice over everything and serve warm.