sometimes one could want the event start and end times to be in 15 minutes instead of 30 minutes steps, which is the default. 

To change this:

edit /ow_plugins/event/controllers/base.php

Search the lines: 

        for ( $hour = 0; $hour <= 23; $hour++ )        {            

$valuesArray[$hour . ':0'] = array('label' => $this->getTimeString($hour, '00'), 'hour' => $hour, 'minute' => 0);            

$valuesArray[$hour . ':30'] = array('label' => $this->getTimeString($hour, '30'), 'hour' => $hour, 'minute' => 30);       

}

Change to:

        for ( $hour = 0; $hour <= 23; $hour++ )        {            

$valuesArray[$hour . ':0'] = array('label' => $this->getTimeString($hour, '00'), 'hour' => $hour, 'minute' => 0);            

$valuesArray[$hour . ':15'] = array('label' => $this->getTimeString($hour, '15'), 'hour' => $hour, 'minute' => 15);            

$valuesArray[$hour . ':30'] = array('label' => $this->getTimeString($hour, '30'), 'hour' => $hour, 'minute' => 30);            

$valuesArray[$hour . ':45'] = array('label' => $this->getTimeString($hour, '45'), 'hour' => $hour, 'minute' => 45);        

}

And you're done. Make a note you possibly need to re-apply the change after an Oxwall or event plugin update, since it will possibly be overwritten.