As long as you do not have any employers, co-authors or publishers who have a claim to the work and deny you permission, you may remix your work and put it under a Creative Commons license. Do bear in mind, however, that a Creative Commons license cannot be revoked. You still do own the copyright, but you have made it open, and you can't close it back up to encourage royalties and sales or to shut down uses of your work that you don't like.
If you may want to turn your work into an OER in the future, it is important to plan ahead and make sure that you negotiate your contracts so that you retain the rights to remix your own work and put it under a Creative Commons license if you choose.
There can sometimes be restrictions on remixing your own previous work to create an OER: