No, it is not OK to send a resume without a cover letter. If the job you’re applying for requires a cover letter, then sending your resume without one guarantees you get rejected.
When a cover letter is listed as optional, you should still send your resume with a cover letter. Including a customized cover letter in your application helps make a positive impression on the hiring manager because it shows you aren’t just sending out bulk applications. Ultimately, a cover letter increases your chances of getting the job, even when you aren’t asked for one.