Saurav - You can able to get the opportunity record which is not modified more than 30 days in a batch apex class and send email to the owner in the batch class. Then using apex scheduler class you can able to run this batch daily to send email to the opportunity owner 

Batch Class

global class sendEmailToOpptyOwner implements Database.Batchable<Opportunity >{

   global Database.QueryLocator start(Database.BatchableContext BC){
     return Database.getQueryLocator('Select Id,ownerID from Opportunity where LastModifiedDate = LAST_N_DAYS:30);
   }

   global void execute(Database.BatchableContext BC, List<Opportunity > scope){
     for(Opportunity s : scope){
       //Write the logic to send email to the opportunity owner
     }
   }

   global void finish(Database.BatchableContext BC){
   }
}

Apex scheduler Class

Then write the Apex Scheduler class, then schedule this class using schedulable interface 

​global class scheduledBatchable implements Schedulable {
   global void execute(SchedulableContext sc) {
      sendEmailToOpptyOwner b = new sendEmailToOpptyOwner(); 
      database.executebatch(b);
   }
}

Follow the below steps to schedule the apex scheduler class

1. From Setup, click Develop | Apex Classes and click Schedule Apex.
2. Specify the name of a class that you want to schedule.
3. Specify how often the Apex class is to run.
   • For Weekly—specify one or more days of the week the job is to run (such as Monday and Wednesday).
   • For Monthly—specify either the date the job is to run or the day (such as the second Saturday of every month.)
4. Specify the start and end dates for the Apex scheduled class. If you specify a single day, the job only runs once.
5. Specify a preferred start time. The exact time the job starts depends on service availability.
6. Click Save.

Thanks,
Karanraj