Minimum _total_ hours are 20. So you could very legitimately work Job A that is 10 hours per week plus Job B that is 15 hours per week, and that is OK as together that meets the minimum _total_ hours. During the 12 month OPT period each individual job doesn't have to be 20 hours. But total combined hours should at least reach 20 per week.

That changes during the 24 month STEM OPT extension period. During that 24 month extended OPT each individual job needs to be full-time (per immigration definition of full-time).