Oyster Card help please

There is no advantage. It is easy to buy it when you get to London at the airport.
Each traveler needs his or her own card.

According to transport for London, you can use a contactless credit card too. Buy you would each need your own card.

https://tfl.gov.uk/fares-and-payments/contactless?intcmp=8257

1) Buy your Oyster card when you arrive, no advantage in buying ahead of time (and may cost you postage as well).

2) You will need two cards, one each. The cards are not personal, you can lend them to other people, but two people cannot use the same card at the same time.

For the tube you "tap in" and "tap out", this opens the ticket barrier, and calculates your fare which is based on the number of zones you go through.
For buses you just tap in, the reader is by the driver. Fares are "per trip", so no need to tap out.

With practice you can use the card and walk through the ticket barrier without having to slow down, just like a real Londoner. Never, never wait until you get to the ticket barrier / get on the bus before searching for your card. That is a good way to annoy the people behind you.

The card reader has a small display which shows the amount left on the card. It only appears for 1-2 seconds, you have to be looking at it. You can refill the card at many shops as well as all tube stations.

Are you aware that Brentford is not near a Tube station? Brentford is a train station served by South Western Railway, on the Hounslow Loop; it is in Zone 4 for your Oyster.

If you buy a Visitor Oyster (each), it costs a smaller deposit than a normal Oyster (£3 instead of £5) but unlike a normal Oyster you can't get that £3 back at the end of its use. You have to pay for postage. I'm pretty sure you still have to load it upon arrival anyway.

If you get a normal Oyster (each, as said above) it is trivial at any machine. The directions couldn't be easier. Pay a refundable £5 and put money on the card and away you go....

I ordered my visitor card ahead many years ago, but it isn't necessary. (You could choose how much you wanted to start moneywise).

I don't know if it's different with a normal (non-visitor) card, but when I overdrew mine (I really had no idea how much money I still had on the card - look fast as mentioned above), I was allowed to ride, and when I topped it up, it automatically calculated the debit in!

Also, I think for rides in Zone 1 & 2, it tops out like a day pass. So 5 pounds for all day. Anything out of the zone will cost extra

When you leave London you can keep your Oyster Card, on your next trip just add money to whatever is remaking on it. It’s so easy, I have around £6.00 on mine, next trip I’ll just add £10.00-15.00 depending on how long I’ll be in London.

You can also just tap the Oyster card at the top off machine before the gates that will show the balance. Another tip, as long as there isn't a long line, is that you can use some of the small value coins that you might accumulate to help top up the Oyster. The machines that take coins will have a slot and you can use coins from 10p on up.

Thanks all for your help!
BTW : we did know Brentford is a bit off the ez transport path, but price for an apt/hotel was fabulous.

The silly reasons my wife and I prepurchased our Visitor Oyster Cards (we go in two weeks).

1 - we will be tired when we get to Heathrow. Our cards have £30 each preloaded so we hit the Piccadilly line running.

2 - it’s a nice design.

3 - the Visitor version gives discounts ...buy a hot coffee at Paul’s before 11am, get a free croissant (for Changing Of The Guards we are going to Piccadilly Circus, walking past a Paul’s to take advantage of this, through Green Park and in time to be early outside Buckingham Palace with our free pastry and a drink)

4 - we like to know we have all these things done before we arrive.