If you're talking about the scale or chord without further qualification, then yes, there's no difference.

As for the spelling of the "power chord" (dyad) consisting of an open fifth, it's typically "D5"; as others have mentioned, this can function as major, minor or ambiguous, but the harmonic context is still a major or minor chord rooted in D, not some subset of another chord with a root other than D.

"D/A" refers not to an open 5th dyad but to a D major chord in 2nd inversion, i.e. with the lowest note being the fifth of the chord.