It was only a little more than a year ago that Lady Gaga was opening for New Kids on the Block, and when she had to cancel her Nassau Coliseum appearance, she was replaced by Colby O'Donis.

Yes, the Colby O'Donis.

Of course, that was before the five Top 10 singles, the 15.3 million downloads, the coat made of Kermit the Frogs, the fake-blood-spattered performances, her audience with the Queen of England, and all the rumorinducing comments about not wearing pants and the reasons for it. In other words, that was back when Lady Gaga was just a singer. Now, she is a full-fledged sensation.

"I would like to thank all my beautiful little monsters and my incredibly supportive team for the most memorable year of my life," says Gaga, after learning her latest single, "Bad Romance," has hit the top of the charts. "To have had such a phenomenal reception to my second album, 'The Fame Monster,' is life-changing. I'm so happy I could die."

Not that Lady Gaga, also known as 23-year-old Manhattan native Stefani Germanotta, is ready to call it quits just yet. She's nearing the end of her "Monster Ball" tour with four nights at Radio City Music Hall, and she's looking to follow "Bad Romance" with a series of new singles from her "The Fame Monster" (Interscope) album.

"What she's done is extraordinarily rare," says Keith Caulfield, senior chart manager and analyst for industry trade magazine Billboard. "Part of Lady Gaga's talent is that she is a chameleon, both in her music and her personas. She's also done all that so fast, in such a short amount of time for five or six singles. Things move so much faster these days, and they require artists to be quicker and more on it and, with her ever-changing looks, she is."

After a slow slog of promotion for her first single, "Just Dance," which took nearly seven months to land at No. 1 last January, things started moving faster, as her follow-up "Poker Face" also hit the top spot in the spring, making her the first artist to reach No. 1 with her first two singles since Christina Aguilera did it in 2000.

But it was her MTV Video Music Awards performance - along with her numerous flamboyant costume changes - that finally separated Gaga from her dance-pop contemporaries.

"That was her signifying moment - when she showed she was really in it to win it," says Maura Johnston, pop music critic for NPR's "Weekend All Things Considered." "That's when she saw all her successes and said, 'I'll run with it.' She's been tireless in her promotion, but there's still a 'what will she do next?' aspect to her appearances. Her persona is this weird dichotomy of authenticity and complete facade."

Though the Kanye West-Taylor Swift incident dominated the post-VMA discussions, including the fallout that led to the cancellation of Gaga's scheduled tour with West, Lady Gaga's wardrobe changes were a close second.

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"She knows how to make a striking statement," Caulfield says. "She is on the mark in knowing how to get everyone's attention. . . . There was also a little bit of glee in the way she did it, like she was saying: 'You thought I was just another dance-pop act. Boy, did I surprise you.' It was as if part of her plan was to infiltrate your brain with this catchy pop song, and then turned it into something grander."

Though the airwaves may be filled with songs that are similar to Gaga's hits "Just Dance" and "Poker Face," her persona has set her apart.

"She used the 24-hour news cycle to her advantage," says Hicksville's Johnston. "That was her way of breaking through the clutter, and she was always working this angle that she might do something crazy. It created this mystery about her."

It's an angle that has been used before, most recently by Madonna and Britney Spears, and one that often comes with a backlash.

"I fear there may be Gaga overload," Caulfield says. "She does remind you of Madonna, always reinventing herself and never being satisfied, and there's a danger with that for going one trick too far. . . . If you try with every song to surprise people, at some point, people may tire of that, or it may be too expected. There's a lot of pressure to keep topping yourself, to always have a new look. You keep doing that, and one day you end up putting out a book where you're naked and photographed having sex with Vanilla Ice."

However, Gaga, who has clearly studied celebrity and its pitfalls, may be too smart for that. "Maybe she's trying to build this defined career arc, and this tour is her one big burst of fame," Johnston says. "Hasn't she compared herself to ['70s glam rock sensation] Jobriath? Maybe she's looking at it like: 'This is it, and I'm just going to ride this wave and release all these singles from 'The Fame Monster' and then get out.' It would kind of be the perfect thing for this era."

Now that would be shocking.

THE MANY FACES OF LADY GAGA IN HER VIDEOS

Like Madonna and Britney Spears, Lady Gaga has tied her changing images to her singles and their videos. Here's a look at what's happened in her videography so far:

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'JUST DANCE'

Peak No. 1

Director Melina Matsoukas

What happens Gaga arrives at an already-dead party and gives it a second wind with her performance. She also rides an inflatable whale and smokes a pipe.

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Craziest outfit The disco ball bra with black miniskirt, accessorized by carrying around an actual disco ball.

Gaga scale GA GA

'LOVEGAME'

Peak No. 5

Director Joseph Kahn

What happens Gaga and her crotch-grabbing gang ride the subway and have a run-in with the cops.

Craziest outfit Lavender leotard with matching leather jacket.

Gaga scale GA

'POKER FACE'

Peak No. 1

Director Ray Kay

What happens Gaga gets out of the pool, has a party and plays strip poker.

Craziest outfit Turquoise bikini with exaggerated shoulders and black combat boots.

Gaga scale GA GA

'PAPARAZZI'

Peak No. 6

Director Jonas Akerlund

What happens Her boyfriend tosses her off a balcony, forcing her to be wheelchair-bound for a time and sidelining her career. To get revenge and to get back in the headlines, she murders him with a poisoned cocktail.

Craziest outfit A metal codpiece/dress and helmet, complete with arm braces to help her walk on her high heels.

Gaga scale GA GA GA

'BAD ROMANCE'

Peak No. 2

Director Francis Lawrence

What happens A day in the life of the "Bath Haus of Gaga" - where she wakes up in a white latex bodysuit, gets force-fed booze and is coerced to dance for prospective johns, who bid on her worth using what appears to be a Nintendo GameCube controller. She goes to the highest bidder - a guy with a half-gold face - but then sets him on fire.

Craziest outfit The silver dress shaped like the solar system narrowly beats out the fur coat with a bearskin rug (complete with head, natch) train over white lingerie

Gaga scale GA GA GA