But some text book define optical isomer as a synonym to an enantiomer. I feel confused about this usage.
It is an obsolete term usually used as a synonym for enantiomer. I was surprised to see that IUPAC consider it a synonym for the broader term "stereoisomer" (
link).
what happen if enantiomer has more than one chiral center and no optical effect(no overall rotation of plane polarized light).
Then it's not an enantiomer. Two enantiomers are related as non-superimposable mirror images. A compound that is achiral despite having more than one stereogenic centre is called a
meso compound. It cannot be one of a pair of enantiomers because it is superimposable on its mirror image.
You might find it useful to think about enantiomer and diastereomer in comparative terms - i.e. two compounds can be related as enantiomers or related as diastereoisomers.