Hi Ron,

Following are the 2 approaches to get the desired result and you can modify the code accordingly further.

To access data from "List" either use a loop or index to get the value of it.

//APPROACH 1
public static String getProjectNumber( String id ) {
    String serialNum = '';

    List<Opportunity> values = [SELECT Id, Name, 
              (SELECT PricebookEntry.Product2.Serial_Number__c, PricebookEntry.Product2.Stock_ID__c,
               PricebookEntry.Product2.RecordType.Name FROM OpportunityLineItems 
               WHERE PricebookEntry.Product2.RecordType.Name = 'Service Body') 
              FROM Opportunity WHERE Id = :id LIMIT 1];

    if(!values.isEmpty()) {      
        for(OpportunityLineItem oli : values[0].OpportunityLineItems) {
            serialNum = oli.PricebookEntry.Product2.Serial_Number__c;
            break;
        }
    }

    return serialNum;
}

//APPROACH 2
public static String getProjectNumber( String id ) {
    List<Opportunity> values = new List<Opportunity>();
    values = [SELECT Id, Name, 
              (SELECT PricebookEntry.Product2.Serial_Number__c, PricebookEntry.Product2.Stock_ID__c,
               PricebookEntry.Product2.RecordType.Name FROM OpportunityLineItems 
               WHERE PricebookEntry.Product2.RecordType.Name = 'Service Body') 
              FROM Opportunity WHERE Id = :id LIMIT 1];

    return values[0].OpportunityLineItems[0].PricebookEntry.Product2.Serial_Number__c;
}

Thanks
DG