Legacy Oops, All Spells! deck
The name Oops, All Spells! comes from a joke with the effects of the cards Balustrade Spy and Undercity Informer. This strategy can also be know as All Spells, The Spy (most common name in Japan), Oops, All My Spells, Landless Spy, or Landless Combo (although the last one is less preferred since it may be confused with Belcher).
Overview
All Spells is a self-mill-based combo strategy built around Balustrade Spy and Undercity Informer.
It uses a lot of free mana, rituals, and cards like the spell-lands Agadeem's Awakening, and Turntimber Symbiosis. To protect the combo in game 1, stock lists play 4 copies of Thoughtseize and 4 Pact of Negation.
It is a very competitive deck that can win as early as turn-1 with high consistency.
The strategy is present in Legacy (and Modern and Vintage after the printing of the spell-lands). Sideboards are focused on beating various forms of graveyard and spell-based hate the deck suffers against.
It is one of the most competitive Reserved List-less strategies and an excellent option for budget-limited players.
The core strategy of Legacy Oops is to mill the player's own deck and trigger Narcomoebas. Then, use them to pay for Dread Return's flashback cost into a Thassa's Oracle to win the game.
Stock Decklist
Creatures (22)
Instants and sorceries (31)
Sideboard (15)
You can find an in-depth primer and sideboard guide here: https://mtgmeta.io/articles/deck-primer-legacy-oops-all-spells/