Can you join the us military with online HS Diploma?

As long as the document is from a State or Federally recognized and accredited educational institution, it doesn't matter if it's online or not. For example, a diploma from "Joe's Online High School Diplomas" wouldn't cut it, but a diploma from an online program administered by an accredited school is okay.

It's similar to an online diploma for a college degree; a BS from DeVry isn't going to get you into the military as a commissioned officer, but a diploma from a known university administering online college courses would. The determining factor is whether or not the place you get the diploma from meets known State or Federal educational standards of accreditation.

You can read more on the subject at the Wiki link below.