iso to img os

Download the desired file
Open the Terminal (in /Applications/Utilities/ or query Terminal in Spotlight)
Convert the .iso file to .img using the convert option of hdiutil:

hdiutil convert -format UDRW -o /path/to/target.img /path/to/source.iso

Note: OS X tends to put the .dmg ending on the output file automatically. Rename the file by typing:

mv /path/to/target.img.dmg /path/to/target.img

Run diskutil list to get the current list of devices
Insert your flash media

Run diskutil list again and determine the device node assigned to your flash media (e.g. /dev/disk2)

Run diskutil unmountDisk /dev/diskN (replace N with the disk number from the last command - in the previous example, N would be 2)

Execute sudo dd if=/path/to/downloaded.img of=/dev/rdiskN bs=1m 
(replace /path/to/downloaded.img with the path where the image file is located;
for example, ./ubuntu.img or ./ubuntu.dmg).

Note: Using /dev/rdisk instead of /dev/disk may be faster.
Note: If you see the error dd: Invalid number '1m', you are using GNU dd. Use the same command but replace bs=1m with bs=1M.
Note: If you see the error dd: /dev/diskN: Resource busy, make sure the disk is not in use. Start the 'Disk Utility.app' and unmount (don't eject) the drive.
Run diskutil eject /dev/diskN and remove your flash media when the command completes