When you add NaOH to water it splits up into sodium ion (Na+) and hydroxide ion (-OH).

The sodium ion finds its way to the negative charge that is left on the oxygen in the oleic acid, and a surfactant (sodium oleate) is created.

OH- plus H+ turns into dihydrogen oxide (HOH, H2O).

When you add NaOH solution (NaOH dissolved in water) to oliec acid, the hydroxide ion steals the acidic proton from the oleic acid.