Heathrow to downtown via Tube

You're confusing Oyster cards with Travelcards. An Oyster card automatically charges you the fare depending on where you enter and leave the system. You don't buy zoned Oyster cards.

If you put a Travelcard on your Oyster card then you would have to pay a supplement for the extra zones.

The easiest is to add some additional funds to the Oyster card when you buy it. You can get all unused funds returned when handing in the card.

Thanks, Frank II. So if I land at Heathrow, put 20 pounds on my Oyster Card, use it to go into the City, it will handle the zones for me?

Yes. The fare from Heathrow to Zone 1 is £5.10 between 6:30 and 9:30 AM and £3.10 all other times.

How much of that is covered by the Travelcard I'm not sure but the Oyster card will figure it out.

By the way, don't say you are going into the city. Say into London. The City is an area of London and people will think that is where you are going. (It's the financial district.)

Frank II, thanks again so much. Since you are knowledgable, will I be able to use my Oyster Card for train from Paddington to Windsor or Oxford?

The majority of tourist sights are in zone one. Not in your question, but, the amount spent on your Oyster card will cap at £7 per day. After that, nothing will be deducted from your card, so you essentially ride for free after reaching the daily cap. That comes in handy when you want to visit a number of places for a short amount of time. If you download the tube map from the tfl.gov.uk website, it shows where the zones are.

Round trip train tickets to Windsor for my wife and I were £15 total a couple weeks ago.

No. Windsor and Oxford are outside London travel zones.

"will I be able to use my Oyster Card for train from Paddington to Windsor or Oxford?"

Oyster cards are issued by TFL (Travel for London) and are valid for travel in London generally, Some exceptions are Heathrow and Gatwick.

"By the way, don't say you are going into the city. Say into London."

Yes and "downtown" is generally not used, always reminds me of Petula Clark. :-)

If you have a Travelcard loaded on the Oyster you can buy paper tickets valid from the boundary of the zone to other further out destinations, eg boundary Zone 2 to Oxford. However not all machines sell these, plus if you only have the inner two zones it doesn't save much compared with a ticket issued from London.

The furthest west you can use Oyster on the main line out of Paddington is West Drayton.

To buy tickets to locations outside the TfL London network, go to the National Rail Web site (www.nationalrail.co.uk) which has all the fares from the different rail operators.

I'm pretty sure lines to both Oxford and Windsor are operated by GWR, in which case you can use the GWR app to book tickets. It's completely digital so you use your phone when going through ticket gates (where present) and if asked by a ticket inspector.

Thank you, everyone for all the help!