Hi Tony,
Left replying to your question a little late, eh...
I came across this old thread, and thought it deserved some sort of follow up, with a few years experience and with three Omega Optical/DGM filters in my kit.
What I have come to understand about Omega Optical is as they manufacture astronomical filters, not just for themselves and DGM, but also for other high profile brands. What is key here is as they have the manufacturing processes on tap, they are also able to experiment to produce exclusive lines and hybrid filters. One of these hybrid filters is the OIII-hBeta filter I first got from them.
This filter is a excellent 2-in-1 filter. It is not a pure OIII, and it not pure UHC, but a mix of the two. An OIII filter will not show, for example, the Horsehead Nebula - the OIII-hBeta does. A UHC isn't much good for planetary nebula - the OIII-hBeta is very good.
I was able to do a comparison between the Omega Optical's OIII-hBeta and their dedicated hBeta filter, and the DGM NPB in a Horsehead Nebula showdown. Scope used was a 17.5" dob. To make things more certain with observations, several other experienced observers also looked through the scope with each filter in turn. All concurred with my observations.
hBeta filter: As expected has the dimmest overall image due to its very tight transmission specs. The Horsehead was visible the most clearly of the three, but man, it is still a tough target. The best way to describe viewing the Horsehead is an exercise in trying to see the slightest variations in black on black. The Horsehead is not seen as a horsey as it is in photos. Instead it is a pimple that bulges out from a black curtain onto an ever so slightly less back background. It requires patience and a few star landmarks to spy out. Once you see it, curiously it becomes much more obvious. Trick is to locate it in the first instance.
NPB filter: Nebulosity surrounding the Horsehead is brighter, and the stars are also brighter. The Horsehead prominence is neither here nor there more obvious than with the hBeta filter. But the NPB has more applications and is brighter than the hBeta.
OIII-hBeta hybrid filter: The overall image is again brighter than with the NPB. The Horsehead itself proved a little more difficult to observe, but it was still there. Just to prove things were not dicky with this filter I tried a dedicated OIII filter and the nebulosity was not detectable, and certainly no Horsehead.
In my eyepiece case there are four filters, the three mentioned above and a dedicated OIII filter from GSO. The filter I use the most is the OIII-hBeta hybrid! I know it is a compromise between an OIII and a UHC type, but for me it gives me the best of both worlds. The NPB and OIII I use when I want a more severe examination of prescribed objects - no getting around that. The hBeta filter is the one I least use - in hind sight I really could have done without purchasing it.
I also have a broadband Deep Sky Filter, and this puppy is nothing like the OIII-hBeta. It behaves more like an OIII filter but with a wider band transmission. I really only use this filter with my smaller apertures. Even from home, the OIII-hBeta gets the most use to overcome light pollution.
If you only want to have the one filter nebula filter that will serve you as a mighty fine all-purpose filter, the OIII-hBeta hybrid is a might fine option. I have found some people just can't come to terms with the concept that this is a hybrid filter. For some folks it has to be either a 'true' OIII or 'true' UHC type, and a hybrid filter just does not compute. But not everyone is so hardcore about things, and if this is you, you will be happy with this hybrid filter.
Mental.
