Fill a large pot with lightly salted water and bring to a rolling boil. Drop in soba noodles and return to a boil. Cook uncovered, stirring occasionally, until tender yet firm to the bite, about 7 minutes. Drain.
Meanwhile, combine soy sauce, oyster sauce, and mirin in a small bowl. Set aside.
Heat olive oil in a skillet over medium-high heat. Add bell pepper, onion, and green beans and saute until softened but still crisp, 2 to 3 minutes. Add soba noodles and toss with the cooked vegetables. Mix in soy sauce mixture and cook for an additional 1 to 2 minutes. Add baby spinach and cook until desired doneness, 2 to 3 minutes. Serve in a bowl.
