Accidentally used dd on my main OSX drive
Sorry to hear of your situation. The first megabyte will have included the drive's partition and critical structural information.

If you can, dd all the remaining content to another drive as a back-up of sorts. Then I suspect your best choice is a professional repair service or a tool like DiskWarrior:


  DiskWarrior
  
  Everything just disappeared after your Mac went haywire. All your work documents. The music you most enjoy. The movie of your kid's first steps. It's your life and it's gone. Don't panic! DiskWarrior will recover your documents, photos, music and any other files when disaster strikes and you lose access to your files.


Recovering a Mac Drive on OS X

@DavidAnderson mentions in the comments below that recovery may be possible. This assumes your drive was GUID partitioned. The Cloudy Aide-Mémoire article, Recovering a non-readable disk on Mac OS X goes into more detail:


  
  Connect your external disk
  Start terminal and use the 3 commands in bold below:
  
  
  diskutil list
  sudo gpt recover /dev/disk1
  diskutil eject /dev/disk1
  
  If the last command was successful, disconnect and reconnect your disk, which >should now be OK
  
  
  An alternative to the second command (gpt) would be diskutil repairDisk /dev/disk1
  
  Note than the manpage for gpt does not cover the recover option whereas the manpage for diskutil does cover the repairDisk option.


Good luck – and be sure to back-up in the future!