and the function name is ossnUNDERSCOREprint ossn_print

(and this editor is driving me nuts) : First ">" is obsolete, line should start with "if"

if (isset($params['title'])) {
    $title = $params['title'];
  if ($title == ossn_print('site:index')) {
 $title = 'YOUR_OWN_GREETING';
} else {
 $title = $title . ' : ' . $sitename;
}
 } else {
 $title = $sitename;
 }

sorry: Father's Day celebrating in Germany - got too many beers already :(

Tried using that to no avail. All I got is a blank page for some reason.

You might want to handle this special case like

if ($title = isset($params['title'])) {
    if ($title == ossn_print('site:index') {
        $title = 'YOUR_OWN_GREETING'; // or better add your custom string to language file
    } else {
        $title = $title . ' : ' . $sitename;
    }
} else {
    $title = $sitename;
}

Yeah, saw that thing. Problem is that the line on page.php is shared by both the news feed and login pages. What I really wanted was to change the welcome message into a different one without affecting the newsfeed title and the like.

see /themes/YOUR_THEME/page/page.php around line 13

I'm referring to the one on the browser title bar, i.e. the title that shows up on a browser tab.

You mean css? or the icons on topbar? and by title bar you mean topbar blue/black bar?