Tatum O'Neals's sensational smile faded as she solemnly explained that her grandmother's current illness was indeed "taking a toll" on her father. Forcing him, it seems, to - yes - grow up.

"My father is actually very mature," Ryan O'Neal's daughter explains, as if she were defending a peer accused of goofing off in study hall. "I know he acts like he isn't, but he's really very intelligent. The problem is that because he got married when he was 20, he feels like he missed out on all of his young years, so he's trying to relive them now.

"And since he's got the face for it - why not?"

"You have to realize," she continues, leveling her straight green gaze, "that we've been together since I was 7. Solidly. So, of course, by now I know him very well. But he is very honest. He has always treated me like a human being, not like a kid. Not like a daughter. We're like equals - like the same person. I feel like I'm his support. I know he needs me."

Bright, polite and professional, Tatum O'Neal at 14 is the picture of sophistication. Her father, seldom far from her side, is absent from her suite in the Watergate Hotel this afternoon because of his mother's illness, but Tatum is unfazed.

In fact, if it weren't for the telltale wriggling, the incontrovertible evidence of an adolescent's shorter attention span, you'd think you were talking to a 34-year-old.

She has not, of course, had time to see everything yet, including - until three weeks ago - "National Velvet," the 1944 movie by which Elizabeth Taylor, as a butcher's daughter who trains a horse to win the Grand National, became a household name at the age of 12.

But then, she has been busy - most recently playing the same sort of role in the MGM sequel, "International Velvet," which had a gala premiers last night at the Kennedy Center.

Tatum O'Neal, of course, is already a household name. At 9, she broke hearts as the cigarette-puffing waif of "Paper Moon." At 10, she won an Oscar for the part and retired. At 11, she came out of retirement to pitch strikeouts for "The Bad News Bears," and followed that by once again teaming with her father for Nickelodeon."

Yet, says Tatum, without so much as a wince of put-on: "I'm really just a simple girl."

That, of course, is not quite true. Tatum O'Neal is frighteningly self-assured for a 14-year-old. "NO," she confirms matter-of-factly, "I'm not insecure. Well, I mean, I guess I am insecure because I'm 14 and I'm still getting my things together. I don't have my . . . um . . . uh . . . pattern down or how I should look or act.

"But I've always like glamor. I remember the day I finished "Paper Moon" I went to New York and met Cher and ooooohhhh . . . there were her nails . . . and her clothes. I met all this style in New York and I loved it. I wanted those high shoes. I wanted the whole thing although I'm sure I'll soon be out of it."

As of yesterday, however, she clearly wasn't out of it yet. On her feet were heels - four-inch beige ones. She was wearing a blue cotton shirt, detailed with the pale green of a matching blouse worn under a white linen blazer. Her short hair revealed two small ruby hearts on each ear, and her makeup, what there was of it - a little eyeshadow, lip gloss, powder - was precise and discreetly done.

Until she was 7, Tatum, along with her younger brother Griffin ("intelligent and interesting - not at all like your normal 13-year-old), lived with their mother, actress Joanna Moore. It was not a happy time, and Tatum opted to live with her father.

It is a period in her life she says she would rather not discuss, even though "it wasn't like they said in the papers. Things weren't . . . easy . . . living with my mother. But I don't remember it actually. I was very young and I just knew I wanted to be in a better household. I wanted to be where things were more stable. I wanted to be with my father," even though, she hastily adds, "it wasn't because I didn't love my mother . . ."

She gently but firmly signals a stop to discussion of that part of her life."Now, she's one of the great ladies I know. She went through a lot of heartache and stuff but she's got her own life now. She lives in Palm Springs and we go to see her all the time . . . lay out in the sun . . . go in the dune buggy . . . things are fine. Really."

That time, though, has apparently left its scars. When Tatum talks about marriage it is always in a wised-up tone: "I've seen too many marriages collapse . . . yes, I think I will get married, but only if it is absolutely the right person because I've seen so many marriages crumble. Now my parents are very friendly but when I was young I'd go back and forth between two divorced parents."

She stops. Turns to the press agent across the room . . . to her friend Carrie Earl sitting in a nearby chair . . . and looks out the window.

Finally, speaking to no one in particular, she adds: "I'll never get married. I don't think I'm going to. I think I'll just be in love and stay in love and keep in love and . . . not marriage."

Children, on the other hand, she thinks would be nice even though she says one needn't necessarily be married to have them. "I think I should have children," she says, adding solemnly . . . but not for awhile."

Indeed. Especially since says Tatum, she is not yet even dating.

"I don't," she says with a simple air of resignation. "I just haven't met anybody I feel like dating. No, Michael Jackson and I are just good friends. Besides, the age to start dating is usually 15. I'm going to be 15 in November," she laughs, "so I can hold out. I'm patient."

And her father - will he mind? "Well," she concedes, "he is very cautious of me going out, and yes, I think he will be very jealous of whomever I date." In fact, bringing beaus home to dad is something she is clearly not relishing: 'You're not kidding." She laughs. She rolls her eyes. She knows.

"I'm really looking forward to that."

If there is one thing that bugs Tatum it is the criticism leveled at her father for the way he is raising his daughter. She, for instance, doesn't mind his assorted lady friends, and says her father has "good taste" in women nor is she the disco habitue the press purports her to be. "I have only been to Studio 54 twice because I wanted to see what it was like. But I rarely go out, so when I do it's bam, bam, bam, bam, bam . . . and the pictures are everywhere."

Now, yes, she does hang out with Cher and Bianca Jagger, but only for shopping and lunches. Her best friend is 16-year-old Carrie Earl who has accompanied her to Washington.

But, still, the criticism hurts. "They were just so awful for awhile. They just kept telling him 'You're not bringing your daughter up right.' But now that things have turned out okay, they can just be quiet."

'He taught me everything I know," she says flatly. "And I don't consider myself dumb.

"But you see," she continues in a patiently lecturing tone, "being who I am, I've had to take on a lot of responsibilities. I've had to be older. That's why I'm so glad my dad brought me up the way he did. And I don't try to get away from him. He lets me do practically everything I want, so I would therefore never do anything bad because he gives me so much trust. I wouldn't want to go out and betray that trust."

And besides, it's not as if they're always together. Sometimes the two have, as she puts it, "big breakers" where they don't see each other, where "he doesn't even feel like he owns me anymore. Like for the last three weeks I was with my friend so much that we barely spoke, until finally he said, 'I haven't even talked to you for three weeks. I don't even know what you're doing. You're not even mine anyone.

But then Tatum insists neither Ryan nor the public need worry: She's not going to screw up. "I know a lot of people are waiting for me to do something awful like OD on drugs or something, but they can just wait. I won't. I'm not crazy. I know where I'm at and I know I'm very much in the public eye, so I can't go out and do something awful." Her tone leaves little room for doubt.

About her own faults, Tatum is, well, candid. She is, she says, selfish. And spoiled. "I'm spoiled because I've gotten a lot. But I also try to give a lot away. I try and go against all I have because I don't want to be spoiled. I do spoil myself with clothes, but then I can afford them.But I am selfish because I want to think of me first even though in some cases Dad comes first. My instincts, however, usually lead me the right way."

Nonetheless, Tatum O'Neal is a star, and she knows it. "I am an actress. I am a star. I can be demanding," she says easily. "But I only demand when it is for the best. I know for instance I have power on movie sets and I try not to use it. But I enjoy being who I am."

Part of them enjoyment, comes one assumes, from having found one's life work at the ripe old age of 14 . . . a decision that eliminates, among other things, says Tatum, the need for higher education. "I'm an actress. Why do I need to go to college?"

The Academy Award for best supporting actress doesn't hurt one's self-confidence either. "How do I feel about it?" she asks, her face lighting up at the memory. "Well, when I got it my heart was pounding madly. I didn't quite know what was happening. But I certainly don't go around sleeping with it, if that's what you mean."

Yet, having made it a 9 can have its drawbacks.

"You mean," she says, her slightly powdered nose crinkling in a delightfully 14-year-old way, "you mean where do you go from the top?

"You stay."