Since March 20, there have been four cases in which men on bikes ride up to their victims and rob them, police said.
Two of the robberies were reported on March 20 in the 2000 block of Telegraph Avenue and in the 900 block of Clay Street at 5 p.m. and 9:10 p.m., respectively, police said.
Two more robberies occurred Friday, at 1 a.m. in the 600 block of Ninth Street and 11:50 a.m. in the 900 block of Oak Street.
The suspects are described as black men between 18 and 21 years old, standing 5 feet 8 inches to 5 feet 9 inches tall and weighing 140 to 150 pounds, police said. They were dressed in black jackets and dark jeans.
There have been no reports of weapons used during the robberies, according to police.
Anyone with information about the crimes is asked to call the Oakland Police Department's major crimes section at (510) 238-3821.
Police remind residents to be aware of their surroundings, especially while on the phone or using electronic devices, and to stay in well-lit and populated areas.
Additionally, people entering or exiting their cars should exercise extra caution, police said.