Will fish oil reduce LDL cholesterol?

You have to be careful with fish oil and medications like aspirin - "Be watchful with this combination.
Medications that slow blood clotting (Anticoagulant / Antiplatelet drugs)Using fish oil with medications that slow clotting may cause bleeding.

Some of these drugs include aspirin, clopidogrel (Plavix), dalteparin (Fragmin), dipyridamole (Persantine), enoxaparin (Lovenox), heparin, ticlopidine (Ticlid), warfarin (Coumadin), and others." - http://www.nlm.nih.gov/medlineplus/druginfo/natural/993.html#DrugInteractions

If you weren't on any medication, then there is good evidence that fish oil reduces LDL cholesterol and is beneficial for heart health. E.g. Australia's Heart Foundation position statement (2008) on fish oil and heart health (https://www.heartfoundation.org.au/SiteCollectionDocuments/Fish-position-statement.pdf) states, "To lower their risk of coronary heart disease (CHD), all Australians should:1. Consume about 500 mg per day of combined DHA and EPA through a combination of the following:• two or three serves (150 g serve) of oily fi sh per week• fi sh oil capsules or liquid• food and drinks enriched with marine n-3 PUFA.“
…. ”Fish that live in cold water are rich in n-3 PUFA—particularly docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) and docosapentaenoic acid (DPA). Alpha-linolenic acid (ALA) is a plant-based n-3 PUFA that has many health benefi ts but does not benefi t cardiovascular health as well as marine n-3 PUFA."

It was once thought that a diet high in fat caused high cholesterol levels. However some natives, like Eskimos, eat diets very high in fat. The fat in their diet comes from cold water fish, seals and other saltwater creatures, and they have very little heart disease. Scientist's began to study the effects of different types of fat, on heart disease and cholesterol.
When Omega-3 Fish oils are consumed on a daily basis, and the consumption of less healthy saturated fats and trans fats, fish  oils lowers the risk of strokes, sudden cardiac death, and second heart attacks. Omega-3 Fish oils reduce the blood triglyceride (fat) levels, raises levels of good HDL cholesterol, lowers levels of all non-HDL cholesterol such as LDL cholesterol.
High-density lipoproteins (HDLs) are used to remove cholesterol from the blood, and low-density lipoproteins (LDLs) are used to carry cholesterol through the blood. Having high levels of HDL cholesterol is good for cardiovascular health, but having “high” levels of LDL cholesterol is not necessarily unhealthful. The culprit here is plaque build up, that gets created when LDL and other low-density carriers of cholesterol become oxidized by free radicals. Free radicals are created by inflammation in the artery wall and the blood.
To reduce the inflammation that causes the free radicals, scientists say the we must lower our intake of inflammatory saturated fats, and increase our intake of Omega-3 and fish oils.
Omega-3 and fish oils can be found in salmon, herring, mackerel, anchovies and sardines. These healthy omega-3 oils are also found in Flax seeds, walnuts, pecans and hazel nuts.
It is recommended that those with out any heart disease should eat fish 2 times a week, in addition to other sources of Omega-3, and flaxseed, canola and soybean oils; pecans and walnuts. For those with diagnosed heart disease should be eating, 1 g of Omega-3 and Fish oils per day, preferably from fatty fish. Omega-3 and fish oil in capsule form could be considered in consultation with the physician. If you need to lower Triglycerides you should be 2 to 4 grams of Omega-3 and fish oils per day provided as capsules under a physician's care. If you intend to take high dosages of Omega-3 and fish oils of more that 3 grams per day you must be under a physicians care. High dosages can cause excessive bleeding.
Your diet should also focus on reducing overall high sugar-starch intake and increase the intake of foods high in heart-protecting fibers, vitamins, minerals, and the polyphenol antioxidants abundant in tea, cocoa, and colorful fruits, vegetables, and wild salmon.
As always get regular exercise when trying to improve overall health.
Omega-3 and fish oils, can help change your cholesterol profile, and reduce the inflammation that causes heart disease. When combined with other healthier dietary and lifestyle choices, it can improve your mental health as well.