Reach the indicated level in Competitive mode to unlock the corresponding item:

Level 1: Chimeran Rage, Advanced Radar, Electrostatic Orb Burst, Prototype Ammo.
Level 2: Hip Pack 1, Hip Pack, Adrenaline Boost.
Level 3: Hybrid, Hybrid 2.
Level 4: Belt Accessory 1, Belt Accessory 2, Ring Of Life.
Level 5: Backpack1, Heatstack 3, Heatstack 4.
Level 6: Ammo Clips, Hip Pack 3, Backpack 6.
Level 7: Heatstack 5, Heatstack 7, Hose Tech.
Level 8: Chest Armor 1, Overload.
Level 9: Hose Attachment, Radio, Backpack 2.
Level 10: Black Ops Suit, Hybrid Variant, Hybrid Variant 2.
Level 11: Backpack 4, Heatstack 9, Arm Accessory.
Level 12: Black Ops Helmet, Heatstack6, Invisibility.
Level 13: Sentinel Suit, Armor 1, Heat Stack 2.
Level 14: Hybrid 3 Head, Hybrid 3 Body 
Level 15: Backpack 5, LAARK, Snake Eyes 
Level 16: Warner and Hawthorne Heads, Backpack 3 
Level 17: Hybrid 3 Variant Head, Hybrid 3 Variant Body, Heatstack 8 
Level 18: Capelli, Armor 2, Heatstack 1 
Level 19: Nathan Hale Head, Nathan Hale Body, Backpack 7 
Level 20: Steelhead Head, Steelhead Body, Backpack 8 
Level 21: Chest Armor, Heatstack 10, Backpack 9 
Level 22: Grenade Vest, Infected Hale Head, Helmet 1 
Level 23: Steelhead Variant Skin , Backpack 10, Steelhead Variant Head 
Level 24: Nathan Hale Converted Head 
Level 25: Decayed Head, Corpse Head 
Level 26: Civilian 1, Helmet 2, Helmet 4 
Level 27: Civilian Skin 2, Blue Hat, Steelhead Armor 
Level 28: Scientist Skin, Scientist Head, Doctor Head, Wrench 
Level 29: Clank Backpack 
Level 30: Leech Skin, Deminsionater.