Squeeze spinach to remove any excess moisture. Combine spinach, cheese, eggs, and oregano in medium bowl. Set aside.
Heat large ovenproof, nonstick skillet over medium heat until hot. Add ground beef and onion; cook 8–10 minutes, breaking into 3/4-inch crumbles and stirring occasionally.
Pour off drippings. Season with salt and pepper, as desired.
Add beef mixture to spinach mixture; mix well. Wipe out skillet.
Combine phyllo strips and oil in medium bowl; toss to coat well.
Spray same skillet with nonstick cooking spray. Place 1/2 of phyllo strips in bottom of skillet; top with beef mixture, pressing with back of spoon or spatula to flatten. Top with remaining phyllo strips.
The pie may also be baked in a 2-quart or 8-x8-inch glass baking dish instead of the skillet. Spray baking dish with cooking spray and assemble as directed in previous step.
Bake in 400 degree oven 15–18 minutes or until beef is heated through and topping is golden brown. Let stand 5 minutes.
Cut into 4 wedges to serve.
Cooking times are for fresh or thoroughly thawed ground beef
Ground beef should be cooked to an internal temperature of 160 degrees. Color is not a reliable indicator of ground beef doneness.
Frozen phyllo dough is sold in a variety of 16-ounce packages.
For packages with the dimensions of about 14-x18-inches, use 4 sheets of the dough; for packages with dimensions of 9-x14-inches, use 8 sheets of the dough to equal 4 cups of dough strips.