If you have two zones (468×60 and 300×250), but most of your site only has the 468×60 zone, then a campaign with companion positioning will work just fine — so long as it is NOT a Contract campaign (that is not configured especially for such a scenario).

To explain:

1. For the first banner on your page, OpenX does not always know how many zones are on the page — e.g. with the JavaScript zone tag, it is impossible for OpenX to know how many zones are on the page when the first zone tag is invoked. So, OpenX simply picks a banner for the first zone based on the relative zone probabilities. All that happens with regards to companion positioning is that when the banner selected for this first zone happens to be from a companion campaign, a flag is set to ensure that all other banners on the page (if any!) will then be from the same campaign.

So, if you have a companion campaign linked to your two zones, but most of the time, there is only the 468×60 zone on the page, then OpenX will happily deliver a 468×60 banner from any campaign there, companion campaign or otherwise.

2. However, when you run a Contract campaign, you have deliver targets to meet. The campaign needs to deliver a certain number of impressions, as per your “contracted” requirements.

OpenX also tries to ensure that the impressions “contracted” to be delivered are shared between the banners in a campaign based on the relative banner weights. That is, if you set up a Contract campaign with a single 468×60 banner and a single 300×250 banner, both banners with weight 1, then OpenX will try to deliver even numbers of banners.

However, it will fail in doing so, as your site will have much less available inventory for the 300×250 zone. As a result, the Campaign will under-deliver every Operation Interval, and although OpenX will try to correct for this, it will always do so on the assumption that each banner should get an equal number of impressions, and every time, this will fail.

To address this, if you have approximately 1/5 as much traffic in the 300×250 zone, simply adjust the banner weights to that your 468×60 banner has weight 4, and you 300×250 banner has weight 1. This way, you Contract campaign would have 4/5 of the Contract campaign’s impressions, and your 300×250 banner will have 1/5 of the Contract campaign’s impressions. That should sort out the deliver issues in this case.

I’m going to add another zone (medium rectangle) on just part of my site and I want the ads to be from the same advertiser (i.e., companion positioning).

Scenario: Let’s say I have advertisers that are guaranteed 10k 468×60 ads per month and only 1/2 of my advertisers have companion ads. Also, the portion of my site showing the new 300×250 ad only gets 1/5 as much traffic as the rest of the site.

My question: Will my advertisers 468 ads still show on the non-companion / 300×250 ad pages so they will still get their guaranteed fill?

NOTE: OpenX really should take some of that investment money and hire you to help write their manuals!