Staying in Oxford - what’s the most efficient/cheapest way to get to Heathrow on time?

The direct bus:

https://www.theairlineoxford.co.uk/oxford-to-heathrow-bus/

Or

https://www.nationalexpress.com/en/airports/heathrow/oxford-to-heathrow

What time is your flight?

What time is your flight?

And what is the flight destination?

What is and easy/inexpensive trip to Heathrow?

If it is an international flight that leaves before noon, traveling the night before is 'the best way to ensure a successful....trip'

Do you mean travelling closer to the airport the night before? If so, where would be a good location? Thank you!

The Oxford airline service referenced above is a 24 hours a day hourly high quality bus service if you look at the timetable, which takes 1 1/2 hours- as fast as any train route.
If the motorway was blocked then there are other pre motorway routes the coach can divert on to. The drivers and their controllers know what they are doing.
Forget National Express- they only book you third party on the Oxford Airline anyway, so book direct with the operator. They don't operate any of their own services on the route.
This is a case where there is no need to change hotels to the airport the night before for early flights, given the service levels on the route.
The train would be cheaper, but two changes en route (or one if you route via London Paddimgton), so this is a case where a little bit more money for a direct and comfortable service is a total no brainer.

Unlike Heathrow - Bath where there are nuances about all the PT options that lead to lengthy discussion (!), "the airline" bus for Heathrow - Oxford is an all round knockout winner. Anything else is way down the convenience scale even if sometimes a little cheaper (at least for a one way).

Do you mean travelling closer to the airport the night before? If so, where would be a good location? Thank you!

What time is your flight? Which terminal?

I am not a morning person. So for me, rather than get up at 4 AM, or earlier, and then have to travel to the airport, I'd rather stay near the airport, get a little extra sleep, and feel better. You may not mind getting up early. We are all different.