I'm one of those cachers who likes to get all the local caches, but doesn't often venture much further afield. At the time of posting this cache, I've found 2497 caches in Oxfordshire, 764 in Berks and 505 in Bucks - nearly 83% of my total is in these three counties. (Edit to add, June 2019 - still 82.3% of my finds in B/B/O!)
I was vaguely aware that the point at which Berks, Bucks and Oxon met was in the middle of the Thames somewhere north of Henley, and today I decided to mark it with a cache, in a spot that is in Oxfordshire by a matter of feet. Buckinghamshire is across the brook and Berkshire is on the other side of the Thames.
If you attempt a couple of the multis in Henley itself, you will again find yourself extremely close to the Berkshire border. Some other favourite Oxfordshire frontier caches:
The Best Ale puzzle on the Wiltshire border
Little Bridges near Bishopstone, a few feet from Wiltshire
The Moulsford Downs series where the path is the Berks / Oxon border for a while, in a beautiful beech wood
Many caches between here and Stokenchurch where the Bucks / Oxon border snakes between the valleys.
If this cache is well received I'll repeat the exercise elsewhere along the edge of Oxfordshire.