London Overground have two types of TVM's. There's the large portrait touchscreens, these have the LUL POM interface, which also allow you to buy discounted travelcards and Bus & Tram Passes in addition to NR tickets. I'd assume these are the same that are used at LUL stations on the Bakerloo line north of Queen's Park and also Gunnersbury and Kew Gardens on the District? The latter are smaller TVM's with a TOC style interface, alongside travel information. You can top up your Oyster card with a debit card with any denomination, but can't buy discounted Oyster products (They sell full price Bus & Tram however). They also sell NR tickets.