To download a specific version of a project, specify the project name, followed by a dash, then the version name. The
-y argument answers "yes" to any and all prompts.
drush -y dl og-1.3 # shortcut for pm-download
drush -y en og # shortcut for pm-enable
You can optionally specify the major version as well...
drush -y dl og-7.x-1.3
Care should be taken to ensure you don't download a version intended for a different Drupal version than what you have installed.