Cut the spaghetti squash in half lengthwise and remove the seeds.
Rub olive oil and salt and pepper inside each squash half.
Put the squash halves cut-side down on a parchment-lined baking sheet.
Bake until the squash is cooked and fork-tender, 30–40 minutes.
Remove the squash from the oven and cool for a few minutes.
Scrape squash flesh into spaghetti-like strands using a fork.
Toss squash strands with marinara in a baking dish.