I'm trying to get Apex to pull a field I created which auto-generates an ID number for every opportunity, but I can't quite get it to work. This is the code I have currently, which is returning this error to me "Error: Invalid field Opportunity for SObject Opportunity "

<apex:repeat var="oppx" value="{!relatedTo.Opportunity}">
    <apex:outputText value="{!oppx.Opportunity_ID__c}" />
</apex:repeat>