"There is a large gap between the care given to those who are seriously sick, and those who suffer from milder problems such as depression or anxiety," Emily Hewlett, from OECD's health division, said in a statement.
"It's clear that the latter receive less care then they should. In that regard Sweden needs to do more."
Only 15 percent of patients suffering from mental illness receive care from a specialist, the report said. It also confirmed Hewlett's statement that those with moderate mental issues often slip through the cracks.
"Unfortunately, mild and moderate disorders are not a priority area in Sweden's mental health strategy," it stated.
Social insurance minister Ulf Kristersson has asked OECD to examine mental illness relating to the workplace. He said he was not surprised by the issues presented in the new report.
"It's reasonable criticism and corresponds with my own analysis," Kristersson told newspaper Svenska Dagbladet.
"We are not adequately equipped when it comes to availability to care, and we frequently prioritize wrongly within primary care."
The report also noted that mortality rates for people with schizophrenia in Sweden had increased by 11 percent since 2006, and the rate had gone up for bipolar Swedes by 21 percent. That's three times the rate in neighbouring Denmark.
"It is possible that it is because Sweden is much better at gathering data," Hewlett said. "But no matter what the cause, it's so startling that one must take a closer look and continue to improve care."
About 23 percent of young Swedes between ages 15 and 24 have a mental disorder, the report revealed, leaving Sweden in the median for cases of mental illness.
About 29 percent of young Norwegians suffer a mental disorder, just slightly more than the rate for the US. The lowest rate was in Austria, where 15 percent had a mental disorder.