Marinara sauce with too much oregano and tomato paste
I think your best bet will be to simply increase the volume of the sauce by adding more whole tomatoes... for ease a can of tomatoes of some sort, either whole (peeled), diced, or crushed; or if you prefer, use fresh tomatoes, which will require some more work. Either way, you may want to strain out the seeds if you don't like them. 

You may have to increase some of the other flavors, too, but I think the only way you'll balance the oregano/tomato paste flavors is to simply make more of the sauce. You can always freeze it if you don't think you'll be able to use it all.

How much you need will really depend on how much sauce you already have. If it's a single jar of prepared sauce, you'll probably be fine with a small amount like a 14.5 oz can or if you have more, you may need to go with two of them.