No, it is not OK to send a resume without a cover letter. The only time this would be acceptable is if the job ad specifically said “don’t bother sending a cover letter” which has never happened in the history of the world.
If the job you’re applying for requires a cover letter, then sending your resume without one guarantees you will be rejected.
When a cover letter is listed as optional, you should still send your resume with a cover letter. We believe that even if the ad states the cover letters are optional, deep down they still want you to write one.
Including a customised cover letter in your application helps make a really positive impression and proves that you aren’t just sending out bulk applications.
In our opinion, there is no excuse for not including one.