Hi Mounika,

Try the following code:
 

trigger TriggerOnCase on Case (after insert, after update) {

    CaseHandler handler = new CaseHandler();
    
    if(Trigger.isAfter){
        
        if(Trigger.isInsert){
            handler.changeOppfield(Trigger.new);
        }
        
        if(Trigger.isUpdate){
            handler.changeOppfield(Trigger.Old, Trigger.New);
        }
    }
}

 

public class CaseHandler {
    
    public void changeOppfield(List<Case> newCases){
        
        for(Integer i = 0; i<Trigger.size; i++){
            
            if(newCases.get(i).Opportunity__c != null){
                
                Opportunity opp = new Opportunity();
                opp.Id = newCases.get(i).Opportunity__c;
                opp.Opcase__c = 'Case status was changed';
                update opp;
                
            }
        }
    }
    
    public void changeOppfield(List<Case> oldCases, List<Case> newCases){
        
        for(Integer i = 0; i<Trigger.size; i++){
            
            if(oldCases.get(i).Status != newCases.get(i).Status && newCases.get(i).Opportunity__c != null){
                
                Opportunity opp = new Opportunity();
                opp.Id = newCases.get(i).Opportunity__c;
                opp.Opcase__c = 'Case status was changed';
                System.debug('Opp is: '+opp);
                System.debug('Opp is: '+opp.Opcase__c);
                update opp;
                
            }
        }
    }
}

The trigger fires on after insert and after update.

On after insert it passes the list of new Cases triggered to the handler method by using Trigger.New (Since these are new records, there won't be Trigger.old in this case.)
On after update, it passes the list of both old and new Cases triggered to the handler method by using Trigger.Old and Trigger.New.

The changeOppfield method then checks if the particular Case is linked to the Opportunity or not. If it is linked and the Case status was changed, it updates the corresponding Opportunity's Opcase__c field.