It is quite a coup to secure the services of Renee Fleming to sing with the Oxford Philharmonic Orchestra. She is the leading soprano of her generation - a singer possessed of a keen intelligence as well as a gloriously rich creamy sound. It is a shame that the concert did not fully showcase her talent to best effect.

It was very much a concert of two halves. Things started pretty well with an energetic performance of the overture to Tannhauser. This is a piece that mixes the grand with the playful to great effect. Marios Pappadopoulus conducted with great elan - conveying a real sense of narrative as well as musical drive. The acoustics of the Sheldonian are not ideal for music of the weight of Wagner and so the brass lacked the real visceral punch that you might want. There were also issues with the articulation from some of the higher strings that robbed us of some of the detail.

I suspect part of the issue may well have been the idiosyncratic rearrangement of the various string sections - it may well have been driven by the logistical challenges of the venue but clearly did not serve the music-making as it should have done.

I must confess that Tristan and Isolde is not of the Wagner operas that has ever held any power for me. I know there are many who are entranced by the famous chord - but the 'Prelude' and 'Liebestod' left me somewhat cold. It may well have been a technically correct performance but it did not evoke any emotional response from me - no goosebumps at all.

After the interval, I was very surprised to see Renee Fleming being lead to a platform at the rear of the orchestra for the 'Vier Letzte Lieder' by Richard Strauss. These are perhaps the greatest orchestral songs ever written - and certainly a favourite of many in the hall.

But we were robbed of the detail and beauty of Fleming's interpretation by placing her behind the orchestra meaning that we were constantly straining to hear the words and how she was colouring them. I know from having heard recordings of her previous performances that she is always alert to the text and it would have been glorious to experience this live. But we didn't get that - and this was a massive disappointment.

Thankfully she seemed to sense the disconnection from the audience and the violins were swiftly rearranged to allow her to come to the front for 2 beautifully sung encore pieces - 'Zueignung' and 'Morgen!' which were the highlights of the evening, with the singer and orchestra alive to the shifting moods of the poetry. If only we had been able to experience the rest of the lieder in this way.

So it was very much a wasted opportunity and one entirely due to poor planning on the part of the orchestra. One of the world's greatest singers should be cherished not hidden at the back of the hall.

This was a much-anticipated night on Broad St, long ago a sell-out, with the great American lyric soprano Renee Fleming in town for one of just two British dates. This was to fulfil an unavoidably cancelled past appearance at the Sheldonian. She was at The Barbican on Wednesday night, and is en route to Monte Carlo, Madrid and then Baton Rouge, Louisiana.

The Sheldonian was abuzz for this programme focusing on two great mysteries of life, love and death, the anticipation palpable - the audience having come from far and wide; my immediate neighbours had travelled from Wiltshire and Derbyshire - the tiered seats bobbing with necks craned towards the playing space even before the appearance of the orchestra. On an occasion like this, those of us up in the cheap seats forget the cramped perches - I won't call them seats - and the paucity of comfort anywhere in the building in order to revel in the vertical viewing experience, akin to a zoologist peering down at a specimen on a microscope slide, while below the audience presses in on the performers so that former and latter seem almost to coalesce.

The 'Tannhäuser Overture', typically for Wagner, is really a symphony in miniature rather than a prefatory glimpse at the musical themes to come in the main body of the opera. Nevertheless, it does encapsulate the major theme of Tannhäuser - the dramatic conflict between sacred and sexual passion. The overture opens with a brooding melody, and then in no time this grows into a massive, brassy anthem underpinned by commentary on it by the string section with insistent semi-quavers. The pace here is vital: Wagner's notation is for 'nicht schleppend, gehended Bewegung' ie not dragging, but continually flowing. Conductor Marios Papadopoulos achieved the perfect balance between weight on the one hand and constant forward movement on the other.

The 'Prelude' and 'Liebestod' from Tristan and Isolde was next. This is a concert version by Wagner himself of the overture and Isolde's Act 3 aria, her mirage of a resuscitated Tristan, and actually pre-dates the whole opera. Miss Fleming was not to sing the aria; she's 57 now, and sensibly wary of over-stretching her voice. I read somewhere that to this end her intention is to forsake the opera stage for good at the end of 2016. Maestro Papadopoulos, swaying laterally on the podium, took the opening bars at a dead-slow tempo, then gradually increased the emotional temperature as the insistent strings were interrupted by throbbing timpani from Tristan Fry and much later augmented by rich harp work. The chords of the finale, as the lovers are united in ecstatic death, billowed over the audience, not a few of whom, I think, were close to tears.

After the interval came Miss Fleming, a cool, elegant figure in a burgundy lamé dress with brown and burgundy cloak. She surprised me by taking up a position on the rear dais alongside the horn section. I was told afterwards that she had no rehearsal time with the orchestra and needed to have visual contact with Mr Papadopoulos. Whether or not this was so, the whole 62-strong orchestra lay between soloist and audience, with its all but inevitable negative effect upon acoustic clarity. That said, the first song in the cycle, 'Frühling' (Spring), immediately signalled her artistry. She picked out the delicate vocal line and soared to the song's high tessitura, demonstrating perfect intonation. The second song, 'September', focuses on the soprano's middle register, and here Miss Fleming's strengths came into view: a rich tone and dynamic sensitivity to Strauss's expansive, elegiac lines.

As we rolled on through the two succeeding songs, Maestro Papadopoulos as always providing the most sympathetic of accompaniment, it was tempting to hark back to the world of Tristan and his paramour in the Celtic legend. The music seemed to flow on inexorably, with no reason why it should ever end, the audience almost in a state of trance. I was reminded of Tennyson's Lady of Shalott drifting down the river past Camelot on her funerary barge, and of the translation of the mortally wounded King Arthur in a barque to Avalon.

Two encores from Miss Fleming, rapturous applause, and then she was away for the late flight to Nice.