It is true that one can OD on VitD. However, it's hard to say who that may actually affect- many people are already low on vitamin D. The easiest way to avoid toxicity is to have ones VitD level checked when starting a new regimen.

Omegas are a little more difficult to actually OD on- the greater harm may be some of the vitamins that are often found in the omega formulas- particularly Vitamin A. Many omega formulas do not have large amounts of vitA now [or often any] as the risk is much higher with Cod liver oil, which is not generally recommended anyway.

One can have some digestive issues when starting them, but that can often be avoided by starting at a low dosage and slowly increasing- which is sensible when starting anything new. There may possibly be links to a slight increase in vulnerability to cancer... but it is unclear whether that is a valid risk, and if it is a lot of recommendations are that for many groups the slight risk [if it is there] would not outweigh the benefits overall.

The higher grade oils are most often the ones that don't have much else in them. Three that come to mind are Nordic Naturals, Omega Brite and Carlson's. I do not work for any of these companies, do not in any way make money for any of them nor am I personally, professionally or financially associated with any of them, just to be clear. I have personally taken both Omega Brite and Carlson's each for a lengthy amount of time and know many people who have had success with Nordic Naturals. I would recommend Carlson's most out of any of them only because they come in a variety of flavours, formulas and amounts and one can often find deals for the products on Amazon so the cost could be lower AND I have personally taken it.

In terms of mood, the EPA:DHA ratio is often what is most important- usually the recommended is approximately 2:1 but sometimes greater.

I didn't actually find much benefit in terms of depression until I started taking 4.5-5 grams a day of carlsons liquid.

It didn't happen over night, and it won't happen for everyone, and I have known people [bipolar] to become slightly elevated in response to fish oil as well.

As with all things, results may vary.