Different cases for translation purposes
_m('your message'); is used for info or warning messages by the system (this will return)
_e('your text','domain'); is used by themes and plugins in most cases (this will echo the result)
__('your text');the default translation function (this will return)
You can see these functions in oc-includes/osclass/helpers/hTranslation.php
<?php _e('Some text','theme_name'); ?>
<?php echo _m('Some text'); ?>
<?php echo __('Some text'); ?>