According to Zaxaa’s support website, if you want to use OptimizePress (OP) as a sales page / membership page creator (because you want to take advantage of OP design capabilities), yes you can use Zaxaa. You simply put your Zaxaa order button on your sales page that is created with OptimizePress.
However, if you want to use OP membership site functionality (using the Optimize Member plugin) with Zaxaa, the answer is you can’t. For this, they provide you with their own WordPress membership plugin called Zaxaa Member (which is free).
You can, however, use OP to design membership pages (because you want to take advantage of OP design capabilities). After you’ve created the membership pages, you would then protect them with the Zaxaa Member plugin.
Here is a link to the original support article:
http://support.zaxaa.com/entries/42345378-Can-I-use-OptimizePress-with-Zaxaa-