As of my last knowledge update in September 2021, the fastest way to gain Hitpoints (HP) experience in Old School RuneScape (OSRS) depends on various factors, including your combat level and resources available. It's important to note that game mechanics and strategies may have evolved since then, so it's a good idea to check the OSRS community and forums for the most up-to-date information.

However, here are some methods that were commonly used for fast HP XP gain as of 2021:
 

1. Training through Slayer: Completing Slayer tasks can be a great way to train HP because you'll be fighting various monsters with different combat styles. This can also provide you with other combat skills and loot.

2. Chinchompas at the Ape Atoll Dungeon: Using Red Chinchompas to kill Maniacal Monkeys in the Monkey Madness II caves is an efficient way to gain HP experience. This method requires a high Ranged level.

3. Barbarian Fishing: This method involves Barbarian Fishing at the Fishing Trawler. It gives you both Strength and Agility experience in addition to Hitpoints experience.

4. Sand Crabs or Ammonite Crabs: These creatures have high hitpoints and low defense, making them ideal for AFK training. Players often use the "1 HP" method, where they equip items to reduce their HP to 1 and then train on these crabs. This allows them to maximize the amount of experience gained before they need to eat.

5. Nightmare Zone: This minigame can be used to train Hitpoints efficiently by selecting bosses that have low defense and high hitpoints. It's best used for higher combat level players.

6. Vorkath: This is a high-level boss monster that can provide good Hitpoints experience alongside other combat stats. However, it's not recommended for lower-level players.

7. Sarachnis: Another boss with moderate Hitpoints experience and relatively easy mechanics. It's a viable option for mid-level players.

8. God Wars Dungeon: Killing minions in the God Wars Dungeon (such as Bandos or Saradomin) can also provide decent Hitpoints experience.

Please keep in mind that efficient training methods may have changed, and new methods could have emerged since my last update. It's always a good idea to consult OSRS community resources, guides, and forums to stay up to date with the latest training methods and best practices. Additionally, consider your own combat stats, gear, and preferences when choosing a training method.

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