When I visited, two years ago I had only the OS map and my intuition to find this one.
As baza says, it's in a copse, which requires you to trespass. I went in winter when the undergrowth had died back and after frightening a least half a dozen pheasants as I paced up and down, I found a monumental stone, now sadly down, and pretty much ready to be reclaimed by the earth. It was undoubtedly the Hoar stone. All mossy and uneven and covered in leaf-mulch and fruits of the forest floor, it's over 10 feet in length. This one is crying out to be re-erected before it becomes lost to us forever, as it surely will be if it is left in it's current condition.
To reach this Hoar Stone, I parked on the A4260 next to a private road leading to Barton Lodge, then walked along the public footpath which follows the private road. Where the footpath turns north, I struck off in a WNWesterly direction through a plantation of young trees. After about 100yds I came across a small mound, only one yard high and about twelve yards in diameter. In a hollow in the middle of the mound lay the 11ft long Hoar Stone.
The two Hoar stones at Steeple Barton, Oxon, are recorded as far back as 1210CE, referring to them as Nordlanglawe, Langlaue and Succelaue, which mean respectively, 'valley of the long barrow', 'the Northern long barrow' and 'the grave of the goblin'. Later 13th century references call one of them Stanlow or 'burial stone', Demnesweye or 'goblin's pathway or track' and Wyrstaneslawe which means either 'Wyrstons' tomb' or 'the Wyr stone at the long barrow', but the reference doesn't distinguish which stone it belongs to.
(SP 45782412) Hoar Stone (NR) (1) A large, flat irregularly shaped sandstone boulder known locally as the Hoar Stone (a), is surrounded by five fir trees at the end of an avenue of beech trees. It measures 3.2m long E-W by 2.7m wide and is 0.8m thick. It has the appearance of a Burial Chamber capstone, but as it is slightly embedded in the ground and there is no trace of a surrounding mound there is no supporting evidence for this supposition. Published survey (25") revised. See G.P.(a) (2)