Here's why:
* Spelling: The word "oil" has the vowel combination "oi", which typically indicates a long vowel sound.
* Sound: The vowel sound in "oil" is similar to the sound of the letter "o" in the word "go", which is a long vowel sound.
So, even though the vowel "o" is a short vowel sound in other words like "hot" or "dog", in the word "oil" it is pronounced with a long vowel sound.