One of Ho-Oh's biggest problems is usually its middling base 90 Speed, but this can be mitigated by using Flame Charge or Tailwind to boost its Speed to more acceptable levels. Sun support from Groudon is highly recommended to power up Ho-Oh's STAB Sacred Fire and Flame Charge and to remove Ho-Oh's weakness to Water-type attacks. Sun also reduces Thunder's accuracy to an unreliable 50%, which can be helpful because some threats rely on Thunder to take out Ho-Oh.
Flame Charge and Tailwind both have their own advantages. Flame Charge's Speed boost lasts until Ho-Oh switches out and allows Ho-Oh to soften up the foe with a light smack before going on the full offensive. It can also break Focus Sashes and Sturdy so that one of Ho-Oh's other attacks can KO on the next turn. Flame Charge is the more consistent option, as the boost stays until Ho-Oh switches out, and Ho-Oh can continue boosting its Speed till +6 with enough free turns. However, Tailwind offers a significantly higher initial Speed boost. For example, Ho-Oh requires a Jolly Nature and 252 Speed EVs just to Speed tie with positive natured base 90 Speed Pokemon that have a Choice Scarf. However, Ho-Oh can outrun max Speed base 108 Pokemon equipped with Choice Scarf while itself running a neutral nature and only 176 Speed EVs after a single use of Tailwind. Tailwind also benefits the entire team; if Ho-Oh needs to switch out, a counter to your opponent's Ho-Oh check can step in and perform a mini-sweep of its own using the boosted Speed granted by Tailwind. However, Tailwind only lasts three turns after being used, which means Ho-Oh cannot sweep for long when more than three faster threats are still around.
Sacred Fire is always the main attack Ho-Oh will be using, and when backed by STAB and sunlight, outright OHKOes many Pokemon in the Uber tier, such as Mewtwo, as well as at least 2HKOing anything that does not resist it. Brave Bird, Ho-Oh's other incredibly potent STAB move of choice, nails Kyogre, Reshiram, and Palkia for assured OHKOs after just Stealth Rock damage. Brave Bird also provides Ho-Oh with a very strong sweeping attack in case the weather is not in its favor, but the recoil can be huge depending on how much damage it does. Roost rounds out the set brilliantly, helping Ho-Oh recover damage incurred from Stealth Rock and Life Orb, as well possibly allowing it to gain multiple Flame Charge boosts or to set up Tailwind repeatedly. While this Ho-Oh is an excellent offensive Pokemon, the loss of bulk is pretty significant, so it can no longer wall more powerful threats such as Calm Mind Arceus and Reshiram as reliably because both +1 Judgment and Draco Meteor OHKO Ho-Oh after Stealth Rock damage. Weaker threats, such as Shaymin-S and Darkrai, can still be checked, though. Although Fire- and Flying-type STABs are a good combination, they leave Ho-Oh walled by Zekrom, Heatran (it needs Toxic or Hidden Power Rock to beat Ho-Oh one on one, however), and Rock-type Pokemon. Earthquake solves this problem by OHKOing Zekrom after it takes damage from Stealth Rock and one layer of Spikes, always OHKOing Heatran, and 2HKOing at worst all common Rock-type Pokemon. However, without Roost, Ho-Oh is extremely vulnerable to Stealth Rock, so solid Rapid Spin support is necessary with Earthquake.
The EVs are fairly simple. Maximum Attack is used to hit as hard as possible, and the Speed EVs enable Ho-Oh to hit 418 Speed after a Flame Charge, which lets it outpace everything below Choice Scarf Heatran. If using Tailwind, it is best to run 176 Speed EVs, which allow Ho-Oh to outspeed Choice Scarf Terrakion with Tailwind up. An Adamant nature is preferred because it affords Ho-Oh a large amount of much-needed power. However, Jolly allows Ho-Oh to outspeed Deoxys-A and tie with Choice Scarf-wielding base 90 Speed Pokemon, such as Timid Reshiram, after a Flame Charge boost and Jolly Max Speed Excadrill in the sand after a Tailwind. If you want to take advantage of Ho-Oh's bulk, a spread of 140 HP / 176 Atk / 192 Spe can be used with an Adamant nature to outrun everything up to Mewtwo after a Flame Charge boost, but this comes at the price of losing some power and Speed. Flame Plate is possible over Life Orb to power up Sacred Fire and Flame Charge if you hate the recoil. Sky Plate can be used to 2HKO bulky Kyogre and Giratina-O with Brave Bird. An alternate EV spread of 248 HP / 252 Atk / 8 Spe lets you act like tank Ho-Oh with the ability to support the team through Tailwind, still giving Ho-Oh enough Speed to outspeed everything up to Mewtwo.
Groudon, with its great bulk and everlasting sunlight courtesy of Drought, fulfills Ho-Oh's sunlight needs perfectly. Groudon can also set up Stealth Rock to help Ho-Oh nab some important OHKOs and is capable of beating many Pokemon that check Ho-Oh, such as Terrakion and Zekrom. Forretress's good Defense and many resistances allow it to fulfill the Rapid Spinning role perfectly; it can even set up Spikes to facilitate Ho-Oh's sweep. While attempting a successful Rapid Spin is hard in Ubers, Forretress does way more than that, as it can also help tank Outrages aimed at Ho-Oh and check certain threats, such as Terrakion with Gyro Ball. If you just wish to delay the setting up of Stealth Rock on your side of the field, Deoxys-S with Taunt is a good choice, as it can achieve this fairly easily thanks to its high Speed and can set up its own Stealth Rock. Magic Bounce users can also delay the opponent from setting up Stealth Rock. Sadly, the two Pokemon with the ability are easy to KO, making them mediocre partners.
The best counter to this set is Arceus-Rock, as it packs resistances to both of Ho-Oh's STAB moves, is bulky enough to take an Earthquake, and can easily destroy Ho-Oh with one Judgment. Ho-Oh also isn't a fan of Kyogre and Zekrom, as the former disposes of Ho-Oh's beloved sunlight and the latter can handle one hit from Ho-Oh and OHKO it with Bolt Strike. Defensive Groudon can also be an annoyance to this set, as Sacred Fire only 2HKOes it while Stone Edge OHKOes Ho-Oh if you fail to burn Groudon. Rock-types are rare in Ubers, and the only semi-common ones are Tyranitar and Terrakion, which are easily checked by Groudon. Thankfully, Groudon's bulk and very powerful STAB Earthquake make it an excellent counter to Zekrom, opposing Groudon, and Rock-types, including Arceus-Rock. Palkia can easily come in on Kyogre's powerful Water-type strikes and defeat it with Thunder. Arceus-Grass is capable of checking pretty much all of Ho-Oh's counters, either crippling them with status or setting up on them with Calm Mind. Latias can also help defeat Kyogre and Terrakion with her powerful Grass Knot.