No, it is not OK to send a resume without a cover letter. If the job ad specifically asks for a cover letter, then sending your resume without one almost guarantees you’ll get rejected.
Even if a cover letter is listed as optional, you should still submit one. Including a well-written, tailored cover letter helps make a positive impression on the hiring manager because it shows you aren’t just sending out bulk applications. It’s also a great opportunity to show some personality and showcase achievements you weren’t able to fit on your resume.