A better way to do this...
Actually, I've found that you should add the wp-blog-header.php call to the "main.inc.php" file, as adding it to "index.php" will only affect the look of the index.php file. So, in the "main.inc.php" file, add the following line with the other includes and requires (which you can find under the line that reads "#include required files"):
require(ROOT_DIR.'../wp-blog-header.php');
Now, the above code assumes that you have osTicket in a folder under wordpress, such as: /home/webuser/html/blog/tickets. "Blog" being the root directory of your Wordpress install and "tickets" being where your ostickets install is.
Now, you also need to change your header and footer files in the include/client directory.
Make sure to keep backups of your original "footer.inc.php" and "header.inc.php" files.
In the "header.inc.php" file, blank the entire file and replace it with this line:
<?php get_header(); ?>
Do the same to the "footer.inc.php" file, but insert this code:
<?php get_footer(); ?>
You may have to tweak your theme in order to get things working (such as moving your sidebar and the related divs to your footer, instead of leaving them in the page). But this gives you a good place to start.