Orange Knight's element is fire. Fire has one hit of damage over time, but unlike poison, pretty much more than half of the enemies are weak to it. Just to name a few, Barbarians, the Barbarian Boss, Trolls, Bears, Royal Guards, Stove Faces, Fish Men, Snakeys, Iceskimos, Ice King, and Cyclops II are weak to fire. In addition to that, fire deals full knockback to all heavyweight enemies! (Stove Face, Royal Guards and Coneheads). Fire Demons resist the knockback, the fire effect, and only take one damage from all fire attacks. Cultists resist all fire based attacks as well, but you can spam splash magic on them to farm XP, by taking advantage of the chain damage they take. Speaking of chain damage, characters with fire magic are excellent boss slayers because fire damages very rapidly like poison, but only two enemies resist it. The XXXY is also excellent because a lot of enemies are weak and the DoT creates extra damage.
That's just stuff about fire in general. Orange Knight (as well as Pink Knight) have the longest splash magic range of any characters in the game. That already gives Orange Knight great crowd control, but unlike Blue Knight he has one of the least expensive splash attacks in the game. The damage over time also contributes to killing enemies faster, and by level 13 with max magic Orange Knight can one-hit kill a barbarian with splash magic, making him good at crowd clearing as well as crowd control.
Damage/Hit: Base Magic Damage x 0.5 + Fire DoT Effect
Creates a horizontal burst of flames. It is the longest splash attack in the game, covering the entire screen (if his magic level is high enough.) The range combined with the knockback make this spell viable for exceptional crowd control. It's also rapidly damaging to bosses, not every hitbox needs to touch a boss in order to hit the maximum amount of times.
Shoots a horizontal or downward angled fireball. A ball of fire trailing smoke. The Fire DoT (although short compared to most characters) effect adds a nice bonus, with the downside being the Fire element meaning some enemies will resist it. It's also among the best projectile attacks for experts. His air projectile is the same as his ground projectile.