The Old School RuneScape combat level is calculated from these skills:
Base level: (Defence + Hitpoints + Prayer ÷ 2) × 0.25
Melee level: (Attack + Strength) × 0.325
Range level: Ranged × 0.4875
Mage level: Magic × 0.4875
Your combat level is determined by the Base level plus the highest of Melee, Range, or Mage levels.