To cure vampirism in The Elder Scrolls IV: Oblivion, you have two main scenarios depending on how long ago you contracted the disease Porphyric Hemophilia (which leads to vampirism):

If less than 72 hours have passed since infection

You can cure vampirism easily by removing the disease before it fully takes hold. Methods include:

• Drinking a Cure Disease potion
• Casting the Cure Disease spell
• Eating food with a Cure Disease effect (e.g., Mandrake Root or Shepherd's Pie)
• Praying at a Wayshrine, Chapel, or Temple

If you do this within 72 hours, you will not transform into a vampire

If more than 72 hours have passed and you are already a vampire

You have two main ways to cure vampirism:

1. Complete the "Vampire Cure" quest

• Start by talking to Raminus Polus at the Arcane University in the Imperial City.
• He will send you to Count Janus Hassildor in Castle Skingrad.
• The Count will direct you to Melisande, a witch living near the Corbolo River.
• Melisande will cure your vampirism but requires five empty Grand Soul Gems to make a Cure Vampirism potion.
• After delivering the soul gems, you receive the cure potion to end your vampirism permanently.
  This quest is lengthy and involves traveling to multiple locations

2. Use the Font of Renewal in Deepscorn Hollow (requires Vile Lair

expansion or included in the remastered version)

• Travel to Deepscorn Hollow, located in the southeastern corner of the map.
• At the Font of Renewal, interact with the pool and the stone pillar after collecting ore from veins on the walls.
• This instantly cures vampirism without the need for the quest.
• Note: Using this method locks you out of the Vampire Cure quest and permanently prevents you from becoming a vampire again in that playthrough

Additional Notes

• Once cured by either method, you cannot become a vampire again without using console commands or mods.
• If you want a quick cure and have the remastered version or Vile Lair DLC, the Font of Renewal is the fastest and easiest method.
• If you prefer roleplaying or want to earn gold rewards, the Vampire Cure quest is the traditional route.
• Console commands can also be used to spawn the cure potion instantly for an immediate cure

In summary, cure vampirism before 72 hours with standard disease cures, or after 72 hours by either completing the Vampire Cure quest or using the Font of Renewal in Deepscorn Hollow for a faster solution