Here's why:

* Oleic acid is a fatty acid: It's a long-chain carboxylic acid, which is generally non-polar and therefore not soluble in water.

* Sodium hydroxide is a strong base: It dissolves in water to form a strongly alkaline solution.

* Reaction, not dissolution: When oleic acid and sodium hydroxide are mixed, they react chemically. This reaction forms sodium oleate, a salt that is soluble in water.

The reaction:

```

CH3(CH2)7CH=CH(CH2)7COOH (oleic acid) + NaOH (sodium hydroxide) → CH3(CH2)7CH=CH(CH2)7COONa (sodium oleate) + H2O (water)

```

Therefore, oleic acid doesn't dissolve in sodium hydroxide; it reacts with it to form a soluble salt. This reaction is commonly used in the production of soap.