@Javoni
I do not think you can, osclass does not allow it.
What you need to do.
Go to file:
/oc-content/themes/tatiana/inc.search.php
Find:
$sQuery = osc_get_preference('keyword_placeholder', 'tatiana_theme');
Replace with:
$sQuery = __('Search', 'tatiana');
And finaly add word Search to translation file.