Rx Java Android Needs to call multiple api's in a sequence and save each results

no, you should use map() or doOnNext(), it will look like this

Observable.just(1)
    .doOnNext(value -> {
        someRequestX().execute();    
    })
    .map(value -> {
        return nextRequestY().execute();    
    })
    .doOnNext(requestYResponse-> {
        someRequesZ(requestYResponse.someValue).execute();    
    })
    .map(requestYResponse-> {
        return someRequesK(requestYResponse.someValue).execute();    
    })
    .map(requestKResponse -> {
        return someRequesJ(requestKResponse.someValue).execute();    
    })
    .subscribe(requestJResponse -> {
       doSOmethingWithFinalResponse(requestJResponse );
    })

First of all, for network requests is better to use Single then Observable, because there always will be only one item. To switch from one requests to another, you can use flatMap.

Assuming your code is similar, you can try this:

class Dto1 {}

class Dto2 {}

class Dto3 {}

public interface Api {

    Single<Dto1> getDto1();

    Single<Dto2> getDto2();

    Single<Dto3> getDto3();
}

private Api api;

public void callApi() {
    api.getDto1()
            .doOnSuccess(dto1 -> {/*do something with dto1*/})
            .flatMap(dto1 -> api.getDto2())
            .doOnSuccess(dto2 -> {/*do something with dto2*/})
            .flatMap(dto2 -> api.getDto3())
            .doOnSuccess(dto3 -> {/*do something with dto3*/})
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe()
}

try doOnNext() or map() method of Observable and use sync execute() of each response and pass them further