with Crossrail, OOC is better linked to the centre of London than Euston is.
As if one Crossrail is a better link than Euston's 4 tube lines. Even assuming people would just take rail, there's a single point of failure with OOC. Euston has 2 lines to the West End and 2 to The City already - which gives redundancy in case of failure, and more capacity than Crossrail gives. The only things OOC has going for it over Euston are direct-to-Docklands and terminating Elizabeth line services meaning a guarantee OOC passengers will have space on trains.
But there's more than just trains!
The Central London Rail Terminal Report has the following modal shares for Euston in 2011:
Underground 53%, Walk 20%, Rail 12%, Bus 8%, Taxi 3%, Cycle 3%.
And distances travelled from the termini: 1st quartile 2.1km, 2nd quartile (median) 3.6km, 3rd quartile 8.6km.
As a proxy for OOC, I'll use Paddington. But Paddington is better connected (SSLs and Bakerloo, as well as more bus routes) and about 3km closer to the centre (so walking would be higher!).
Modal share: Underground 62%, Walk 12%, Bus 7%, Rail 6%, Cycle 4%, Taxi 4%
Quartile distances: 1st 3km, 2nd (median) 4.6km, 3rd 7.2km.
Now coming in and taking rail back out to a suburb (or airport in the case of Paddington) is at play at both termini, screwing up that top quartile, but it's clear that Euston is not only tubeable to the centre, but closer to the action and walkable in quite a lot of cases - unlike OOC!
OOC and Euston compliment each other well - but Euston will take 60%+ of HS2 passengers for a reason.
Yes. Because of Crossrail it's quicker to get off at OOC for Oxford Street, the City, Docklands, Heathrow, Stratford and the Olympic Park.
Docklands, Heathrow, Stratford and the Olympic Park yes. However none of them are "the centre of London", which was your earlier point.
So let's look at "the centre of London". Journey times from OOC (taking a minute off Acton ML times) vs Euston to:
Bond Street/Oxford Circus: 8min vs 2min
Tottenham Court Road: 10min vs 4min
Farringdon: 13min vs 6mins
Moorgate/Liverpool Street: 16min vs 8mins
So Euston is 6-8 minutes less on TfL-run services, but that's going to work out a little bit slower when you factor in the change time and the OOC-Euston time.
But we've done the places with direct access from OOC. We've not done Theatreland, the Royal Parks, Victoria, Waterloo, the southern City, London Bridge - all of which are central and all of which have direct trains from Euston, but not OOC.
Lets look at something a change from both: South Kensington (museums, Albert Hall). It's not really the "centre of London" but it's a popular destination for out of towners and your claims of where OOC is better for keep changing anyway.
From Euston: 14 minutes (change at Victoria)
From OOC: 24 minutes (change at Paddington and I'm additionally cutting the wait time for a Circle line train down so it's a 5 minute change at Paddington, not a 9 minute change)
OK, crow-flies distance from OOC is 3.7 miles, while Euston is just 2.9 miles away from South Ken. But Euston's 78% of the distance takes just 58% of the time because Euston is better connected. Adding the change points to the crow-flies make it 4.5 and 3.6: An additional 0.8 and 0.7 miles, and so it's not like the journey from OOC is particularly convoluted - its that it doesn't connect to the fast N-S lines with a single change.
As I've just said, it's quicker to OOC from most of London than it is to Euston.
point being made changed yet again!
Unless you're going to/from a small part of Camden, anyway.
Or Lambeth, Southwark, the rest of Camden, the south of the city of Westminster, etc, the south of the City of London, North London, South London....