I tell her how much I like one of the paintings that adorn her walls, of some shoes with golden hair falling over them. "Yes, Magritte was quite good," she says, as if we've all got one at home.

We go into the kitchen to talk over coffee. At first she seems distracted, taking phone calls, organising a trip to Brazil, where a massive Yoko Ono art retrospective and a concert are due to be held. She is just back from Iceland, where, on the anniversary of John Lennon's birthday, she created the Imagine Peace Tower - possibly because peace was always their joint idea. Peace doesn't just keep the world alive: it keeps John and Yoko alive.

She studies me closely, like an artist sizing up the proportions of a still-life. Suddenly she says: "I love the way you've got one tooth sticking out. That's beautiful." I tell her it's the result of my childhood fear of the dentist and my determination never to wear a brace. It's an odd observation, but then she continues staring at me. "Really," she says eventually, "you wouldn't be attractive at all without it."

Right from the start, you realise that Ono is a one-off. She sees the world in a different way from everyone else. She tells me I would have loved the Peace Tower, which she unveiled on Videy island, near Reykjavik. The tower is a beam of light radiating skywards from a wishing well. She says she was proud of the speech she wrote, which was all about hope. One passage says: "The light is the light of wisdom, healing and empowerment. Even in the moments of confusion, fear and the darkness of your soul, hold the light in your heart, and you will know that you are not alone, that we are all together in seeing the light of peace..."

Ono often comes across as mystical and wise, but also she can be hugely sensible. She was worried about the Iceland unveiling because it had been raining so hard. "I thought everyone was going to slide into the mud. It had been raining for several days, but when I arrived it was beautiful and on the night there was misty rain, which was perfect. If that air had been too clean, there'd be nothing to hold on to, so it was a kind of magic. I really felt it was a message to the universe."

She believes in the universe taking care of things. I tell her that her skin is fantastic: there are no criss-cross lines and no saggy bits. It's the same Ono who peered out from the cover of The Ballad of John and Yoko. "I don't know about that," she says. "It used to be good but I've been having only four or five hours of sleep lately and I don't think I'm in very good condition. But I could be. All of us can become younger."

What exactly does she mean - metaphorically or physically? "Metaphorically as well, but I meant physically, because these days, if we want to be healthy, we all know what to eat. We all know what to do. I don't eat meat, not even fish much. Just vegetables, carbohydrates, very light on your body, easy to digest. Sportsmen in the old days used to train for the Olympics and just have spaghetti."

When she was a child, Ono was sent to a music school in Tokyo for super-toddlers. Her father, who had been forced to give up his career as a pianist and become a banker - as stipulated in his father's will - wanted her to be a concert pianist. She wanted to be a composer. They compromised and agreed that she would be an opera singer, which helped her to become a musical maverick, a performance artist. She met Lennon in 1966 at one of her conceptual art shows, where he had to hammer in an imaginary nail. "In a way, both John and I ruined our careers by getting together, although we weren't aware of it at the time," she once said. She led him in to experimental music and he led her further into him.

During her life as a Beatles wife, she was reviled by press and public alike, and her music and art were not taken seriously. But a younger generation listens without prejudice, and her last album Yes, I'm a Witch, which came out earlier this year, received excellent reviews. So, she's going to Brazil to do a big concert. She seems rather bewildered by the plaudits. "It is amazing. I was not a very popular figure and I got used to that. Whether I'm popular or unpopular, what I do is the same and I just kept on doing it."

Indeed, there seems to be a new wave of Ono reverence. When I met the film director John Waters, creator of Hairspray among other films, he said the best thing about being single was that when he went to one of Elton John's parties "all the other guests were in boring couples" and he got "the best seat in the house - next to Yoko Ono". She giggles when I tell her this. "Mmm, yes. There were a few interesting bachelors around that table." Is that a euphemism for gay? "I never question whether people are gay or not," she says ambiguously.

She is putting her wisdom in a book which, she says, is filled with "recipes for life". But it's only being published in Japan. "I did one a while ago for children, and this one is for grown-ups. It's a how-to-deal-with-life kind of book." I imagine she'd be very good at relationship problems, I tell her. "For some reason I am. I'm into healing. Sometimes things just come into my head and I write them down." Has she any psychic powers? "Well, I think so," she says. "I remember once, in London in the 1960s, a man I didn't know very well asked me to predict his future. I said, 'The next five years are going to be very hard for you," and then I forgot all about it. Years later, I bumped into him again and he told me I'd been right. 'What about my next five years?' he asked. And I said, 'Oh, it's going to be so very good.' But what did I know? I only told him that because I thought he would make it good. I never wanted to take responsibility for predicting bad things - if they think the future is going to be bad, they might deliberately make it bad."

Does she believe that if you worry about something, you almost make it happen? "Yes, I do. I'm always trying to make things happen in the right way. John once went to see a famous psychic who told him he was only going to be here for another five years. John was very upset. He had all this guy's books and he threw them in the bin. Everything was going really well for John and me at that time. We never thought it would end. Maybe we should have said we wanted this to continue forever."

She stops, a little breathless, and then draws a parallel between her life with Lennon, being told he was going to die, and Sleeping Beauty. "In that fairytale, an old psychic told the king, 'Your daughter is going to die. Someone will prick her with a needle.' So he decided that no one in the kingdom should have a needle. But, of course, when the princess was older she went into the tower and there was a witch who was weaving and she pricked her finger. So even though the king had outlawed needles, they slipped in somehow. We were living our lives being very careful and he slipped in with his bullets."

Lennon still seems very much present in her life. She is surrounded by his pictures, his words, his music. She said she felt him with her in Iceland. "At one point he was right there next to me. He does come in and out, you know - he doesn't always stick to me."

The first time she thought she saw him after he died was "one day when I was half-asleep. I kept being attracted by something at the window. Suddenly I thought, 'Oh, it's John!' and he turned round and said, 'Come on, we've got to go.' And I said, 'I can't come with you now because I have Sean and so much to take care of.' He said, 'Oh yes, I forgot,' and just turned round. And I said, 'Later, not now.' And he said, 'Okay.' It was amazing. Like Cathy in Wuthering Heights, scratching at the window to get to Heathcliff. I do see him once in a while."

Only once in a while? "Okay, a lot of times. I never used to believe in any of that stuff. I was a logical person. But since John's passing, I believe there's something going on."