Omega 3s (usually from salmon/fish oil or flax/vegetable oil) will slightly increase your bleeding potential. Nowhere near enough to stop taking your warfarin, as one poster had suggested.
I don't know if it will show up on your INR. Some anticoagulants will, others won't. Check Al Lodwick's site for that.
These oils can be good for smoothing out arrhythmias (as can magnesium). Again, I wouldn't drop any prescriptions because of them, unless your doctor agrees. We have had a couple of people who were able to go off of prescriptions because of magenesium, but it's not very common.
The only class of people who shouldn't take these oils are those with specific forms of heart failure. Both types of oil work at the 2gm/day level, but the fish oil starts working sooner, builds up levels more effectively, and lasts longer if you miss a few days.
Best wishes,