Excadrill is a stellar lead on HO. It's an effective rocker with Mold Breaker and also a great spinner as it threatens every Ghost-type in OU, providing useful role compression which is needed on a fast playstyle like HO. Earthquake is strong STAB and Rock Tomb allows it to keep up rocks against Tornadus-T.
Dual screens Tapu Koko is an amazing support mon on HO. Taunt prevents the opponent from defogging Drill's rocks and also prevents Koko from being setup fodder. Screens allow its teammates to come in and setup much more safely. U-turn is there to pivot into those teammates obviously.
Hawlucha is an obvious partner to Koko as it benefits greatly from its Electric Terrain, considering it activates its Electric Seed which in turn activates Unburden and in addition increases its defense to even higher levels. Swords Dance boosts its mediocre attack to more respectable levels, while Roost aids this setup process by providing longevity. Dual STAB in HJK and Acrobatics are Hawlucha's strongest attacking options.
Mega Gyarados is a common Dragon Dance sweeper on HO. Intimidate is amazing as it weakens opposing mons allowing for easier setup. Mold Breaker is incredibly useful as it allows the team to beat Unaware users and hit Rotom-Wash with Earthquake. Crunch provides amazing neutral coverage and hits Psychic-types that Hawlucha might have trouble with super-effectively, while Earthquake provides good coverage and hits common Pokémon like Magearna and Toxapex hard. Substitute is chosen because Gyarados hates being inflicted by burn, paralysis or toxic, easing its matchup against stall. Waterfall can also be chosen over Substitute as the team is lacking Water-type attacks.
Magearna is a great sweeper with formidable snowball potential thanks to Soul-Heart. It is also a good partner to Koko as the terrain boosts its Thunderbolt. Shift Gear allows it to outspeed most of the metagame and Calm Mind allows it to setup to break past would-be checks. Its final and Z-move is Flash Cannon which allows it to break past things that Thunderbolt can't, mainly bulky Grass-types and AV Magearna.
Finally we have Greninja as our scarfer and cleaner, as well as a secondary hazard setter. Rock Slide allows it to revenge kill offensive mons like Volcarona and Gyarados which could otherwise form a great threat to this team. U-turn is for momentum, pivoting into teammates like Koko on passive mons allowing it to renew terrain and/or screens. Ice beam OHKOs Lando which is otherwise dangerous to the team and also takes care of Zapdos which would otherwise check Hawlucha and Gyarados. Spikes wears down opponents allowing its teammates to sweep more easily.