<aura:application>
    <aura:attribute name="accId" type="String"/>
    <aura:attribute name="oppId" type="String"/>
    <c:CreateRecord accountId="{!v.accId}" opportunityId="{!v.oppId}"/>
    <aura:dependency resource="markup://force:*" type="EVENT"/> <!--When I didn't include this, it errored-->
</aura:application>

<aura:component>
    <aura:attribute name="opportunityId" type="String"/>
    <aura:attribute name="accountId" type="String"/>
    <aura:handler name="init" value="{!this}" action="{!c.doInit}"/>
</aura:component>

({
    doInit : function(component, event, helper){
        var createRecordEvent = $A.get("event.force:createRecord");
        createRecordEvent.setParams({
            entityApiName: "Custom_Object__c"
            , defaultFieldValues: {
                Opportunity__c: component.get("v.opportunityId")
                , Account__c: component.get("v.accountId")
            }
        });
        //These are being populated correctly
        console.log(component.get("v.opportunityId"));
        console.log(component.get("v.accountId"));
        createRecordEvent.fire();
    }
})