OU Tier Explanation

The following general rules apply to all tiers and all matches. The breaking of any single rule, including the match clauses, at any time is grounds for immediate disqualification. If your opponent breaks a rule in your match, you are to report it to the host. Breaking any of the rules may result in a penalty of three points being deducted from your season record. If you are given a 3 point penalty, you are not allowed to join subsequent tours the same week in which the penalty is given. Bad faith attempts to circumvent rules or instances of dishonesty or unethical behavior are dealt with on an individual basis. A range of penalties may be applied in such situations. Habitual rule breakers will be barred from joining the League permanently.
In the case of a player disconnecting during a battle, he or she is automatically assumed to lose unless the non-disconnecting opponent chooses to grant a rematch. If you can return to the battle before the timer ends, then you can continue battling.
No coaching, advising, or consulting between players is allowed during the League. Anyone found to be either giving or receiving advice will be disqualified and penalized in the standings. Collaboration on team building before the tournament is acceptable.
Do not play matches using someone else's account with or without permission

Select this clause when challenging for all League matches. This is necessary in the interest of finishing the tournament in a speedy manner. Makes use of the Pokemon Showdown's timer. Must be activated at the beginning. No intentional timer-stalling.
A player cannot have two of the same Pokémon on their team. For example, a player cannot have two Koffing on his or her team. This also includes alternate forms. For example, you can't have both Deoxys-D and Deoxys-A on your time, or Rotom-S and Rotom-F.
A player cannot put two or more different opposing Pokémon to sleep using attacks that induce sleep to opposing Pokémon. The enemy is allowed to put their own Pokemon to sleep, even if one of their other Pokemon is already asleep, through the use of rest.
A player cannot increase their Pokémon's evasion stat with a move that specifically increases evasion. Items or indirect boosts do not break this clause. Legal methods of increasing evasion are the item Brightpowder, the move Accupressure, (which has a chance of increasing evasion) and the ability Contrary when hit by an evasion-lowering move.
Not that anyone uses those anyway
Players cannot use moves that have a chance of instantly KO opposing Pokémon. For example, Horn Drill is an illegal move to have on a Pokémon's move set.