From the Guidelines

Fish oil has a beneficial effect on cholesterol levels, particularly in reducing triglyceride levels.

The American College of Cardiology expert consensus decision pathway notes that prescription-grade omega-3 fatty acids, including mixtures of EPA and DHA, have been shown to lower very high triglyceride levels at a dose of 4 grams per day 1.
The American Heart Association recommends eating fish, particularly fatty fish, at least two times a week, as it is a good source of protein and low in saturated fat, and provides significant amounts of EPA and DHA, which are cardioprotective 1.
Studies such as JELIS and REDUCE-IT have demonstrated the benefits of EPA alone in reducing major coronary events and cardiovascular death, with a significant reduction in triglyceride levels 1.
However, it is essential to note that fish oils may increase plasma LDL cholesterol, so monitoring is required 1.

Omega-3 fatty acids have been shown to decrease the risk of arrhythmias, thrombosis, and heart attack, as well as decrease triglyceride and remnant lipoprotein levels, and improve endothelial function 1.
The anti-inflammatory and hematologic effects of omega-3 fatty acids may also contribute to their beneficial effects on cardiovascular disease 1.
The American Diabetes Association recommends limiting saturated fat to 7% of total calories and minimizing trans fat intake, and suggests that two or more servings of fish per week can provide n-3 polyunsaturated fatty acids and reduce the risk of cardiovascular disease 1.

From the Research

Fish oil has been consistently shown to lower triglyceride levels, with studies reporting reductions of 28% 2, 32% 2, and 27mg/dL 3.
The effect of fish oil on low-density lipoprotein (LDL) cholesterol is more controversial, with some studies reporting increases of 3% 2, 12% 2, and 6mg/dL 3, while others found no significant change.
High-density lipoprotein (HDL) cholesterol levels may be slightly increased by fish oil supplementation, with studies reporting increases of 1.6mg/dL 3 and 0.01mmol/L 4.
Total cholesterol levels are generally not altered by fish oil consumption, with studies reporting no significant change 2, 4, and 3.

The dose of fish oil consumed may influence its effects on triglyceride levels, with higher doses associated with greater reductions 4 and 3.
Baseline triglyceride levels may also influence the effects of fish oil, with greater reductions observed in individuals with higher initial levels 4 and 3.
The type of fish oil supplement used may also affect its efficacy, with ethyl ester and triglyceride forms of fish oil producing different results 2.

Fish oil supplementation produces a clinically significant reduction in fasting blood triglyceride levels, but has minimal effects on total, HDL, and LDL cholesterol levels 4.
The reduction in triglyceride levels is dose-dependent and correlated with the intake of EPA and DHA 4.
The increase in LDL cholesterol levels associated with fish oil consumption is generally small and may not persist with long-term use 5.