You can have the PageReference method do both. Can you attach a code snippit to add more clarity?

You need to set your Save button's action attribute to a method in your controller.  Are you using the Opportunity standardcontroller and a controller extension, or a custom controller?

If you're using an extension then your controller probably looks something like this.  For the purpose of illustrating how this would work, let's say your VF page is used for the purpose of creating some custom object named Services related to the Opportunity, and then you want to email the Sales Manager that it was created.

public with sharing class OpportunityControllerExtension {

            private final Opportunity opp;

            public Services__c service      { get; set; }

            public OpportunityControllerExtension( ApexPages.StandardController controller ) {

                         opp = ( Opportunity ) controller.getRecord();

                         service = new Services__c( Opportunity__c = opp.Id );

            } 

            public PageReference extensionSave() {

                         try {

                               insert service;

                         } catch( System.DmlException e ) { 

                               // do something here

                         }

                         Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();

                         email.setSubject( 'New Services Record Created' );

                         email.PlainTextBody( 'A new Services record has been created. <a href="...../" + opp.Id>Click Here</a>' );

                         email.setToAddresses( new String[] { salesmanager@acme.com } );

                         Messaging.sendEmail( new Messaging.SingleEmailMessage[] { email } );

                         return Page.Success;   // given that you have a VF page named Success that displays a message

            }           

}

On your VF page, set your Save button's action attribute like this.

...

<apex:commandButton value="Save" action="{!extensionSave}" />

...

Hope that helps!

~ Clint