Lady Gaga and Jennifer Lopez obviously believe they should go big of go home!

And that, they did ... Gaga donned custom Alaia -- the designer's first-ever Oscars design. The singer will perform tonight and wear ... count 'em, no fewer than THREE custom pieces by Azzedine.

Gaga posed with an unlikely pal -- the more understated Keira Knightley, who wore an embroidered tulle and silk, high-waisted dress over her baby bump. The look is Valentino ... perfection!

The biggest surprise was Jennifer Lopez in a fuller-than-full Elie Saab gown covered in sequins, looking like a hot, Latina fairy princess!