Combine dried oregano, 1/2 teaspoon salt, and pepper in a small bowl; rub evenly over flank steak. Let stand 30 minutes.
Combine avocado and next 3 ingredients in a food processor; process until smooth. Add remaining 1/4 teaspoon salt and mayonnaise; pulse until blended.
Place steak on grill rack coated with cooking spray; cover and grill 8 to 10 minutes or until desired degree of doneness, turning occasionally. Remove steak from grill, and let stand 5 minutes. Cut steak diagonally across grain into thin slices. 5. Place steak slices on toasted baguette pieces; top each with 2 teaspoons avocado spread. Reserve remaining spread for another use. Garnish with oregano sprigs, if desired.