Do you want to offer free advertising for a limited period of time, or just to certain advertisers?

The first option is doable with a small change to one line of code in one of the OIO files.

For the second option, the only viable way to do it right now is to get those particular advertisers to submit their ad, but ask them not to make a payment - you can then manually mark the ads as "paid" from the admin area.

If you open the "include/purchase.php" file, look for this line of code near the top of the file:

var $free_item = 0;

Change to:

var $free_item = 1;

The only thing to bear in mind is that this isn't a "user ready" setting. You will still need to mark a price in the admin area (eg. $0.01) in order to have the ads available for purchase and the user will have to select a payment processor, but they will be taken straight to the thank you page instead of to the processor's website.

Fixed the code as shown in comment above to :
var $free_item = 1;

One problem however.
Although the emails are working to both admin and customer,
after clicking Submit and Make Payment the page goes to
http://www.domain.com/ads/payment.php?do=success&rand=l-VP5s9JvYknbut the page keeps refreshing in the browser window.
The pages shows 'Make a payment' at the top.
It never gets to the thank you page at all?
Any info how to fix this problem much appreciated!

Sorry, I hadn't noticed you made another reply.

Looks like the function was slightly faulty. If you download OIO again from here and just upload the "include/purchase.php" file again (making the same change), it should show the correct message on the thank you page.