String endpoint='https://login.salesforce.com/services/oauth2/token';

String username = 'USER NAME HERE of X org';
String password = 'PASSWORD without security token';
String ClientId= 'connected app on Z org';
String ClientSecret = 'xxxxxx';

Httprequest req = new HttpRequest();
req.setMethod('POST');
req.setHeader('Content-Type','application/x-www-form-urlencoded');

req.setBody('grant_type=password' +
      '&client_id=' + ClientId +
      '&client_secret=' + ClientSecret +
      '&username=' + username +
      '&password=' + password
   );
req.setEndpoint(endpoint);
Http http = new Http();
HttpResponse res;

try {
    res = http.send(req);
    system.debug('body:'+res.getBody());
}catch(system.CalloutException e){
    system.debug('error'+e);
}