ABOUT:
■ You are probably aware of の as a possessive marker. の can also be used as a pronoun. In this case, it could be translated as "one."
HOW to use:
■ When the context is understood, replace the noun with の.
EXAMPLES:
赤いのが欲しいです。
I want the red one.
[red | one | want]
先週見たのを買った。
I bought the one (I) saw last week.
[last week | saw | one | bought]
For a more in-depth look at using の as a pronoun, please click here.