Buy something. I don't have a free solution for OE.
Mozilla?
"Unlike Netscape, Mozilla does not come with a spell checker. Since the spell checker that is bundled with Netscape is from another company, and is a closed source project, it cannot be included with Mozilla.
You can, however, add a spell checker to Mozilla easily, by installing an open-source spell checker at Mozdev.org. This project provides an "XPI" (cross platform installable) file. Several versions of the XPI are available, depending on which Mozilla version they work with. First determine which Mozilla version you are using (by going to Help| About Mozilla), and then click on the appropriate XPI link. You will be asked to confirm that you want to install a piece of software. Click OK to proceed. Once it has been successfully downloaded and installed, restart Mozilla. Open Composer, and you should see the Spell button added to the toolbar:"