Oxford to LHR

That would probably be too risky for me, considering you would have to be at Heathrow by 9 am. The Airline bus, which I think is the best mode of transport between the two, will get you there by 9, leaving at 7:30 am. But that doesn't take into account delays caused by road conditions, traffic, etc.

If I did leave in the morning, I would probably be more inclined to take an earlier bus, like 5 or 6 am, just to be safe. The Airline bus pretty much goes 24 hours a day, so you can pick and choose your times.

Of course, another (and better, imo) option would be to spend the evening in Oxford, have dinner, drinks, whatever, then take a late Airline bus to Heathrow and get one of the airport hotels there that is nearest to your terminal. That way you could check in late at your hotel, but sleep in a bit in the morning. That would be more to my liking.

Here's a link to the Airline bus timetable: https://www.theairlineoxford.co.uk/wp-content/uploads/2024/11/Airline-booklet-sept24-v3-web.pdf

Everyone should do what they are most comfortable with. If it were me personally, I think this is fine. I will put it this way- if I lived in Oxford, I wouldn't spend the night at Heathrow for a 12pm flight. You are fine to arrive two hours ahead, but definitely build a cushion in when you select your bus. It will also make a big difference which day of the week you are going. If it's a weekday you will hit rush hour traffic, but that early on a weekend day the roads should be quiet. A potential alternative, if you are worried about traffic, is get an early train into Paddington and out again on the Heathrow Express. It takes longer but you are less likely to face disruption. Good luck with the planning!

With all the work on the HS2 depot which keeps causing unexpected issues, plus other factors the train is equally risky.
I've just come up from Cornwall overnight and arrived at Paddington 135 minutes late after the sleeper knocked down a suicide with fatal consequences
at 10.15 last night.
The train has huge recovery time in its schedule so in reality we were closer to 200 late.
Tragically that kind of thing can happen anytime.

But my day today has had to be rewritten completely. There is no flight thankfully but still inconvenient.

It's the first time in many years I've been on the actual train concerned, and I believes the first time the deceased has had to be recovered past my train due to the local geography.

On the bus you are always advised to allow 1 hour on top of airline check in time so I would catch a bus at around 6am.

Personally I wouldn't like to be a distance away from the airport for a flight at 12. Especially given the official advice from Heathrow is to allow 3 hours before any long haul flight (I assume the flight is back to the mid west?). I would do as Mardee suggests and spend most of the day in Oxford, take a late bus to the airport and stay at one of the hotels close to your terminal. I certainly would not be leaving a departure to 7am or 7.30am on the day of a flight.

I agree that getting the morning bus is a bit risky. Yesterday, the M40 - the main highway between Oxford and London - was shut for hours after a collision at 8.15am caused a truck to overturn which then spilled fuel. This could happen any day and could be disastrous for your plans.

Evening bus to LHR and stay there would be much safer in my view.

This all seems wise and re-looking at things, maybe keeping Oxford as a day trip as initially planned. Thanks everyone of words of reason.

We were staying in Oxford for 4 nights last June so chose not to transfer the night before our 10am flight as logistics with our daughter were easier to stay put. We got the 5:50am bus from Oxford Westgate shopping center, arrived at LHR at 7:30am, and had no issues. But being so early in the morning on a Sunday probably helped with less traffic. But as others say - there is always a risk of traffic/accidents/etc that could be a problem. So you can chose what your risk tolerance is and make the best choice for you.