Victoria Station to Oxford

The Overground is just a train that rides above ground, rather than below. You just go to the correct platform, board it, and ride it to your destination. Easy peasy. :-) But if you're visiting a friend, you might want to check with them to see what the best way of getting there is, depending on exactly where they live.

With regards to Oxford, the bus is certainly a good way of getting there, especially since you are staying near Victoria Coach Station.

The Oxford Tube is a good way to get there, with modern high quality double deck coach-buses. They have only recently got a wholly new fleet.

The Overground is part of the TfL network with through fares, even if you have to go through intermediate fare gates like at Liverpool Street. £2.90 each way off peak or £3.50 peak. Nice modern new frequent trains. The Overground has gone from being a neglected part of the London transport system to a key, modern part.
And often unique views of the street scape.
The through fares apply as long as you don't hang around at an interchange, but take the next train out.
I enjoy riding those trains.

You want either Clapton or Hackney Downs station, although it looks like quite a long walk from either. Get the Circle Line to Liverpool Street, and then take a Weaver Line overground train with "Chingford" as final destination.

It's only around half a mile from either Hackney Downs or Clapton stations, maybe less depending on exactly where on Lower Clapton Road you want. Easily walkable unless you have mobility issues.

Very helpful, thank you all, this is exactly the type of suggestions I am looking for!

if you are not in a hurry the venerable old route 38 bus goes from Victoria to Hackney, traversing the length of Lower Clapton Road with frequent stops.

Cheaper than the tube or Overground, considerably slower, but get an upstairs seat, especially at the front and the world's your oyster for the journey.

The 38 to Clapton Pond (I've always loved that name) leaves Victoria Bus Station in front of Victoria Station (NOT Victoria Coach station) from stand "D".

All the stops and stand letters and maps are at https://tfl.gov.uk/bus/route/38/?Input=38+towards+Clapton+Pond&direction=inbound

Nigel, thank you for the tip about the bus! On an earlier trip to London, I took the boat out to Greenwich then rode the bus back into town, and it was delightful! Even though the bus ride was a bit long, it was nice to be in the midst of everyday London life with the locals, and school kids. Sounds like a bus to Hackney might be the ticket! :-)