An OEM package is basically the same thing as a normal managed package; OEM vs. ISV is about who's providing licensing. With the OEM licensing model, you create contracts with salesforce.com for the licenses, and your clients create contracts with you. With the ISV model, users already have a salesforce.com contract, and form a separate licensing agreement with you. OEM installations have additional restrictions in place, so you'll want to read more about OEM User License Comparison.
To get started, you'll need to sign up as a partner in order to get it "blessed" as an OEM package. Once your application has completed the partner process (including getting a security review), your package will no longer count towards configuration limits. The Partner team will help you with the process and give you additional information, including how to set up Trialforce, which gives you the ability to create trials that are already pre-configured with your package.
You can use a normal managed package, but you'll probably want to upgrade your developer org. Once you're approved for the partner program, you can request that your developer account get upgraded to a partner developer account, which has more storage, more licenses, and higher limits, or you can create a new org from the portal and then create your package there. Once that you can't use the namespace you already used if you follow this path.