There are definitely pro’s and con’s to every supplement. As they say, everything causes cancer and everything prevents cancer.
I can quickly find two studies indicating the benefits of fish oil in relation to cancer:
Fish oil improved weight, appetite, reduced post-surgical morbidity, and results in improvement in clinical, biological and QoL parameters for cancer patients.
Fish oil decreases the risk of colorectal cancer: "Significant dose-dependent reductions in risk were associated with increased consumption of omega-3 polyunsaturated fatty acids"
Beyond those cancer-related benefits of fish oil, there are far reaching benefits associated with the omega-3 fatty acids, DHA, and EPA in fish oils related to depression and mental illness, cardiovascular disease, and weak claims about Alzheimer’s and dementia.
On a side-note, I highly recommend this online visualization/database:
It categorizes health benefits associated with a variety of dietary sources by the strength of the study: strong evidence, good evidence, promising, inconclusive, slight, and none.