4 hours before serving (or the night before) cut the strawberries in half. Place half the berries on a baking sheet, and freeze.
Combine the orange juice and the remaining strawberries and refrigerate.
Just before serving, combine the orange juice mixture and the champagne. (For a non-alcoholic version, substitute gingerale or sparking white grape juice).
Add the frozen strawberries, which will act like ice cubes without watering down the mimosa.