MMUI which stands for Miva Merchant User Interface is the standard user interface file that comes with Miva Merchant shopping cart.  OUI is an add-on so if you did not install it you do not have it.

What is OUI?

OUI is a layer that interacts between you and the standard Miva interface (mmui) and functionally replaces it.  OUI is a free download from openui.org and is the product of Starbase21.

What are the pros of OUI?

It is easier to build modules for so you will find some modules that will only work in OUI. The ones that will work in both OUI and MMUI may require you to edit the MMUI.mv file (not all modules require this and usually module coders give detailed instructions as to how and where to edit the file for their module to work).

It will enable you to control your page layout to an very high degree using cheap add-on modules.  To get the same layout control using MMUI you will need to make code modifications to the mmui.mv file, which may be simple or extensive depending on the kind of page layout you want.  

If you are using mmui.mv, incorporating your design is easily done through the admin interface of the Miva Merchant shopping cart using HTML (provided that the overall page layout options in Miva Merchant are to your liking).

While Miva Corporation WILL NOT support a store using OUI there are many coders who are experienced and can offer support.

What are the cons?

Since it is an additional layer it makes the site slower. Depending on the number of modules added and other variables, it could be a little bit slower or a lot slower.

Another thing to maintain, get support for, and potentially screw-up on your site.  Additionally you will have to upgrade OUI each time (or almost each time) you perform minor or major updates to your Miva Merchant shopping cart.

Two modules using the same OUI components (hook-points) may conflict with each other.  If you decide to go with OUI you will need to check with the module suppliers and ensure that your new module will not conflict with another module you already own.

Since it takes over the mmui.mv, you will not be able to "hack" into the mmui.mv for quick fixes that does not require a module and that you may want to do just for your store... e.g. insert an external affiliate network code (as in this step by step example http://mivahelp.com/affiliate.htm) or changing the text that says "company" for example in the checkout screen to say organization, or referral source, or whatever else you may want it to be. Such minor modifications are not worth paying for a module and can be easily modified by editing the mmui.mv file.

Final Note:

If you like Merchant the way it is from a functional standpoint as well as the overall layout (you can easily incorporate your design by putting HTML code in the admin section), and you either don't need add-ons (modules) or need just one or two, then stick with the MMUI.  If you think the software is grossly lacking and there are a variety of layout changes you have in mind then OUI may be better for you.