Rick Mather, the Oregon-born, London-based architect who died three months ago aged 75 was celebrated in Oxford yesterday, first at a memorial in Keble College, where he had designed two of his finest new buildings, and afterwards at a reception in the Ashmolean Museum, which he transformed and flooded with light in one of the most dramatic "rearrangements" of our time.

For theatregoers, Rick's innovations are far-reaching indeed: his master plan for the South Bank underpins the brilliant recovery that is ongoing, while his re-design of the Lyric Hammersmith, while slightly hampered when the funds dried up half-way through, remains a thrilling re-ordering of the interior public spaces, and the terrace; his realisation of the Lyric's £13.5m capital project above the adjacent shopping mall will create the UK's first teaching theatre for the performing arts, with rehearsal studios, a recording suite, a cinema, seminar rooms and offices.

The Ashmolean Museum
The Ashmolean Museum
Past and present administrators at the Lyric - Sue Storr and Jessica Hepburn - joined a great throng on Keble lawns and inside the Ashmolean as Rick's praises were sung by his younger brother, Don; the bursar of Keble, Roger Boden; the art historian, fairytale analyst and closest of all Camden Town friends, Marina Warner; his civil partner, David Scrase, deputy director of the Fitzwilliam Museum in Cambridge; and Christopher Brown, director of the Ashmolean.

One of Rick's latest projects - all of which will be seen through by his practice, led by Stuart Cade and Gavin Miller - is a new 150-seater theatre, and new kitchens, in my alma mater, Worcester College. David Scrase tells me that there's a slight hiccup here which means that the work may not be completed in time for the college's 300th anniversary celebrations next year; there was indeed no sign of anything much happening when I popped my nose in the gardens and grounds yesterday morning, although there is scaffolding right along the inner facade of the library facing the quadrangle.

We were on our way to lunch in the Ashmolean in the fourth floor restaurant with Emma Brown, Bill Kenwright's former PA, now married and living in the area and working as Christopher Brown's (no relation) PA. She's finding a little more use for her art studies degree than she did over eight years of working for Bill, though she loved every minute and wanted to keep abreast of all the gossip and, of course, football news and pre-season speculation.

The restaurant has all the usual Mather qualities of light-filled airiness, linear cleanliness and spatial harmony, and it is nothing but sheer pleasure to sit there, above the rooftops of the Randolph Hotel and St John's College and the rolling countryside beyond. There's also a decked outside terrace, with tables, chairs, shrubs and greenery. The food's quite good, too.

Marina Warner described Rick's style as radical, with unstinting grace, while an old Oregon contemporary and fellow architect, Margo Grant Walsh, recalled a fateful walk along the South Bank in 1965 when, even way back then, Rick talked about opening the riverside to the water once more, without any frills or jokes that characterise so much modern architecture; he was a discreet and stylish minimalist, said Walsh, much as Coco Chanel was a dress designer or John Cage a composer.

"He seemed to think with his hands," said the Keble bursar, likening any Mather building to a Mozart symphony, its utter simplicity disguising the genius behind it. This quietness and reserve were all part of an old world American charm, though Rick loved to gossip and giggle as much as anyone. A lifetime vegetarian, he would insist on serving his guests meat, if they wanted, with an hospitable flourish of, "Would you like some dead flesh?"

Before the Ashmolean, Rick Mather Architects was established in the adaptation of three London museums: the Dulwich Picture Gallery, the Wallace Collection and the National Maritime Museum in Greenwich. The Ashmolean, though, is his masterpiece, doubling the display space of the the original neo-classical structure in a six-story extension with a bridged atrium and a grand new staircase: for the Pimms and champagne reception, the entire premises were thrown open to guests, who could wander round this light-filled glass and layered city as though living participants in the display themselves.

The party had already started in Keble, on an inner lawn in that forbidding Victorian Gothic brick pile that Mather's buildings do so much to both soften and complement. There, I found the deputy head of Stowe who told me about the new girls' dormitories Rick had supplied; my old friend Richard Jarman, formerly deputy director (to John Drummond) of the Edinburgh International Festival and now busy with the Benjamin Britten estate and the Aldeburgh Festival; the Booker prize-winning novelist Alan Hollinghurst; the British Museum curator, my neighbour Frances Bindman; and countless others from all walks and disciplines.

Rick and David had restored a dark and ruined cottage on the south coast of France between Nice and Toulon, a light-filled airy vessel, said Marina Warner, which seemed to float out over the garden towards the sky and sea. We saw glorious pictures of it in a slide-show that accompanied the memorial.

For Rick loved plants, and gardens, as much as anything. He had transformed his own rooftop garden in Primrose Hill into a magical oasis in the sky and, as we walked from Keble back to Beaumont Street on this unforgettable Oxford afternoon, my taller and better half reminded me of the yucca Rick gave her many years ago which has suddenly blossomed to a condition of unprecedented beauty, as if marking his demise. Plants, like people, fade and die. But great architecture leaves a lasting legacy.