If you're taking flaxseed for the oil, then yes, you should be taking the oil or possibly the ground form (though note that flaxseed isn't a good source for omega 3s, as very little is actually converted to a usable form by the body). If you're taking it for constipation, you need the whole seeds. They swell in the digestive tract, providing bulk, and they also develop a mucilaginous (slimy) coating which helps move them and everything else along. The swelling is why you need the extra water. You won't get this effect from the oil, you need the bulk provided by the seeds. Ground flaxseeds seem to be popular but I can't speak for them personally: I can't stand the flavour and the whole ones work for me, so I just use those. If you use ground flaxseeds, apparently you should grind them immediately before eating to avoid rancidity. The whole seeds don't provide any nutritional value as they're passed undigested, you're taking them for a different purpose here. You can also get prescription products which do a similar job, but in my experience are full of weird additives and taste peculiar. Some people like to put the flaxseed into liquid, which can include orange juice or breakfast cereal, and let it swell up before they take it. As far as I remember, it takes longer to consume as there is substantially more of it after swelling, plus you notice the flavour more. Anyway, apparently the dose is up to 3 tablespoons daily, though work your way up and see how you get along.
I get constipation as a side effect from some of my meds (not ones I take full-time, thankfully) so I've had to learn how to deal with it. If I'm just taking painkillers for a few days, I will use extra magnesium supplements. If I'm taking painkillers for longer, that's usually when I break out the flaxseed.
Of course, what complicates the issue is that constipation can also be caused by insufficient fat in the diet, and that's probably why some people are recommending flaxseed oil.




