As many companies run into issues with lead source and Sales Changing this on opportunities I have been looking for a Trigger that will allow me to auto populate it if it is blank. I plan on removing from the page and allowing the trigger to update it as it will always be blank when an opportunity is created. 

Are sales Reps are forced to create an opportunity from the Contact which gives us the Opportunity Contact Role, ORC. I have the trigger working with lead source but I was asked to add a custom field in it as well and I think I am missing something. Here is the code that I am using if anyone can help that would be great.  Custom Field Name is Lead_Source_Name__c. 

trigger fillLeadSource on Opportunity (before update) {
   Set<Id> oppsToFill = new Set<Id>();
    for(Opportunity o : trigger.new){
        if(o.LeadSource == null) {
            oppsToFill.add(o.Id);
        }
    }

    // Now we'll select all possible contacts wasting only 1 query.
    if(!oppsToFill.isEmpty()){
        List<OpportunityContactRole> roles = [SELECT OpportunityId, Contact.Name, Contact.LeadSource, Contact.Lead_Source_Name__c
            FROM OpportunityContactRole
            WHERE isPrimary = true AND Contact.LeadSource != null AND OpportunityId IN :oppsToFill];

        if(!roles.isEmpty()){
            for(OpportunityContactRole ocr : roles){
                Opportunity oppToBeFilled = trigger.newMap.get(ocr.OpportunityId);
                System.debug('Changing lead source on ' + oppToBeFilled.Name + ' from ' + oppToBeFilled.LeadSource + ' to ' + ocr.Contact.LeadSource + ' (thx to ' + ocr.Contact.Name + ' being the Primary Contact).');
                oppToBeFilled.LeadSource = ocr.Contact.LeadSource;
                oppToBeFilled.Lead_Source_Name__c = orc.Contact.Lead_Source_Name__c;
            }
        }
    }
}