A No KO Custom Stage is one confined by objects which cannot be passed through, where it is generally impossible to be KO'd or SD. These types of stages provide a different twist on the normal setup, as players fight continuously in an inescapable area, without re-spawning and with high damage percentages. They would generally only be used for matches with various externally generated goals, such as being the first to reach a certain amount of damage, or having the least damage at the end of a time match, and are also conducive to the more traditional Stamina mode. A no KO stage might also be used simply for sparring without the need for recovery.
A standard/typical No KO is composed of a square/rectangular area of basic blocks, with spikes along the top to prevent characters from spawning there. Variations include triangular shape to cause a slant for items, hindering item usage as soon as they spawn, a more rounded stage to provide an aiming challenge, made of icy blocks, and a roof of drop blocks.
As the stage requires at least one space between the stage border and the blast line, there are several Final Smashes that allow players to cross this barrier and be KO'd. Ike's Great Aether sends he and his victim to the peak of the stage, causing a Star KO outside of the main play area. Lucario's Aura Storm also sends him to the peak, which may cause him to SD, and Pikachu's Volt Tackle allows passage through solid objects, and may culminate outside of the intended arena. Snake's Grenade Launcher and Olimar's End of Day also puts them to the stage peak after the FS is over as they come back down from the top of the screen.