Yes — fish oil (omega-3) and vitamin D work in different ways and are commonly taken together with no known direct interaction, but mind the doses (especially with cod liver oil) and check with your pharmacist or doctor.
These are two different nutrients with separate roles, and authoritative sources do not flag a dangerous interaction between them; in fact, they've been studied together (the NIH VITAL trial, which the NCCIH references, tested vitamin D and omega-3 supplementation). The NHS list of medicines and supplements that interact with vitamin D (colecalciferol) does not include fish oil or omega-3. Because vitamin D is fat-soluble, it is generally taken with food. The one real catch is overlap: some "fish oils" are fish-liver oils (like cod liver oil), which the NCCIH notes "contain EPA and DHA, and they also contain vitamins A and D, in amounts that vary from product to product," so you could unintentionally double up on vitamin D — and the NCCIH warns that "vitamins A and D can be harmful in excessive amounts."
Check labels for hidden vitamin D: if your fish oil is a fish-liver oil (e.g., cod liver oil), it adds vitamin D (and vitamin A), so count it toward your daily total. The NHS advises: "Do not take any other vitamin or mineral supplements that contain calcium or vitamin D without talking to your doctor first." Keep vitamin D within the adult tolerable upper intake level of 100 mcg (4,000 IU) per day; far above that can risk toxicity (hypercalcemia — symptoms such as nausea, constipation, excessive thirst and urination). For omega-3, the NCCIH says side effects are usually mild (unpleasant taste, bad breath, heartburn, nausea, diarrhea). Separately, omega-3 at high doses is commonly flagged as a bleeding-risk consideration, so if you take a blood thinner (e.g., warfarin), have a bleeding disorder, or have surgery coming up, it's worth confirming with your clinician — ask your own pharmacist or doctor, as the cited consumer pages here do not cover that specific point. Note the NHS also lists several prescription medicines (heart, diuretic, steroid, epilepsy, and antifungal medicines) that interact with vitamin D itself. Anyone who is pregnant, breastfeeding, has kidney disease or high blood calcium, or takes any of those medicines should get individual advice. When in doubt, ask a pharmacist or doctor and follow the dose on the label.
This is general reference, not medical advice, and not a guarantee of safety. Interactions depend on your doses, health conditions, and other medicines. Always confirm with your pharmacist or doctor before combining products, and follow the dosing on each label.