Welcome to HGSS OU, the holy land of the 4th Generation competitive metagame.
The Physical/ Special split has just taken place and tiers are going through heavy changes.
New threats are popping up from every corner and everything under the sky (and above) is a worthwhile member if you know around the ropes.
Here, there are no team preview and no permanent sun/ rain, making the lead the most important member of every team.
Setting or preventing Stealth Rock, maintaining an offensive presence, tanking hits, and going kaboom: Leads had to perform specific functions to get its trainer ahead of the opponent.
Set-up sweeping for late-game and Choice Item smashing: One could take many directions while playing HGSS OU.
Of course, Pokémon from Unova and Kalos do not exist in this metagame, neither does any items/ Abilities/ moves/ mechanic changes from these times.
Pokémon numbered from 494 does not exist in 4th Gen.
All the Rotom Appliances have Electric/ Ghost typing.
Weather summoned by Sand Stream and Snow Warning lasts forever.
Hidden Abilities did not exist. Hence, most Pokémon will lack their third Ability.
Steel-types resist Dark and Ghost moves.
As Mega Pokémon were introduced in 6th Gen, they do not exist in 4th Gen.
Same goes for the existence of the type Fairy, as it was introduced in 6th Gen too.
Certain moves (like Knock Off) have different mechanics. Research to know more about them.
No WiFi Clause. Hence, players cannot view each other teams and decide which Pokémon goes first.
The Pokémon put at the top of the party in the teambuilder will always be the Pokémon to start the battle.
Tier: When making a challenge, select the tier as Gen 4 OU, keeping the following clauses and banlist in mind.