Nail puller.

[daddo]

Threw all the nails in a bowl. Not counting a few that broke off, 283 nails. My hands are tired.

[WoodworkerTom]

Cat's paw has always worked well for me. One or two hits with a hammer and it is under the nail head and pry it up.

[NWOhioJim]

I helped my brother take up some underlayment nailed with ring shank nails.  We used a garden spading fudge .  Worked real well on the OSB, came up in fairly good sized pieces, then just pulled any nails that didn't come with it.  But the particle board in the bath came up in small pieces. Worth a try if you have one available,

Jim