Yes, it is ok and common to do.
Even if the conference publishes the proceeding, it is still ok. You have then to pay attention, however, to not commit plagiarism between the proceedings text and the article.
Proceedings are usually more like an extended abstract, thus they don't count as full publication. You have to be careful to not reuse the same words in the article (alias: write anew), and declare in the acknowledgements that part of the material has been previously published in the Conf XY proceedings.
W