Yes, it is true that we don't add oil to the boiling water.
I'm not aware of any good reason to waste extra-virgin olive oil that way!
Some oil is always added at the end, over the sauce, when the pasta is already in the plate!
It has to be raw, so that it retains its fruit nuances and texture.
If you are doing cold pasta salad and want to avoid sticky pasta, adding oil in advance won't help.
You have instead to wash pasta under cold water to wash the starch away (you can put the pasta in the colander directly under the running cold tap).
cooking.stackexchange.com/users/16/Wizard79
Alton Brown covered this on an episode of Good Eats.
There is a legitimate reason, and it has nothing to do with sticking; it's an anti-foaming agent, so you don't have to stir as much to keep down the foam you'll sometimes get.
Any oil will work, it doesn't have to be the good stuff.
cooking.stackexchange.com/users/67/Joe
Using it anti-stick apparently does NOT work.
(Just use a large enough pan and stir somewhere during the start of boiling).
Adding oil will also reduce flavor-uptake from any sauce.
cooking.stackexchange.com/users/8457/Tobias Op Den Brouw
I've never put oil on spaghetti's water and i've never seen anyone doing it here in Italy.
To keep your spaghetti out of trouble (sticking), just have the patience to mix them on the water (just for the first couple of minutes).
cooking.stackexchange.com/users/1603/systempuntoout
The most common reason that I have heard is to help prevent the pasta from sticking together (and maybe to add a little flavor).
This is unnecessary on both points, however.
Spaghetti is one of those foods that should be served right away.
If it is, you will rarely have a sticky mess.
If you do let it sit until it sticks, the pasta can be rinsed to loosen it up.
Adding oil to the water does not add much flavor or anti-sticking power.
Most of it will be discarded with the water, and what is left may help prevent the pasta from taking up the flavor of the sauce.
Update:
Joe's answer about oil being an anti-foaming agent is a great tip!
cooking.stackexchange.com/users/1476/SturdyErde
I never used to use oil in the water when I was cooking spagetti but I found that it tended to stick together in a pick clump that was hard and difficult to eat.
I was advised to start using oil in my water
and I have not looked back since.
It keeps the spaghetti separate and therefore an overall better eating experience.
As for oil, it does not have to be olive oil, it can be any kind of oil...or at least that is what I have found!
cooking.stackexchange.com/users/450/Princess Fi
I have always been told that adding fat to the pasta water coats the pasta in the fat, which prevents the pasta from absorbing the sauce.
I have tried it a couple of times.
The results weren’t disastrous (it was edible), but the pasta did indeed seem to shy away from the sauce, so I don’t do it any more.
cooking.stackexchange.com/users/65650/Just Joel
In my opinion, all else being equal, Italian style water-boiled pasta tastes bland.
But I have grown eating pasta and rice boiled with virgin olive oil, so I am certainly biased.
Even after washing with cold water to use for salads as Lorenzo advises, I can tell the difference.
Why, I ain't sure, but I think it might be due to the oil interacting with the starch.
I have also noticed less sticking, but that's not the main reason for me to use it.
cooking.stackexchange.com/users/None/
Normally, I cook pasta without olive oil.
But due to timing differences (I arrive home time X, my wife Y, and the kids Z).
To make sure the pasta doesn't stick, and can be prepared beforehand, I cook it in salted and buttered water for taste.
I stir it continuously to avoid sticking, but after draining the water, I add some EVOO to the pasta as it cools and stir it.
I find that hot pasta tends to stick as it cools.
cooking.stackexchange.com/users/54381/Mike Georgiades
In Italy we indeed put a bit of oil in the boiling water, but ONLY in case of fresh homemade pasta, not in the DRY one that you buy in the supermarket.
In case of fresh pasta, we use flour as anti-sticky, but when put in the water flour doesn't work anymore and a bit of oil helps.
In case of dry pasta is not needed anymore because the causes of stickiness are different.
Beware that you should stir the pasta in the boiling water at least when you put it in and every 2 minutes.
Anyway, stirring to much the fresh pasta can break it (since has different consistence and "physical properties").
Enjoy your pasta!
cooking.stackexchange.com/users/81063/Pierfrancesco Beneventano
Many people (at least in Germany) think that Italians add olive oil to the boiling water for spaghetti. However, Italians tend to tell the opposite (from my experience).  So, can you please state objective reasons for or against adding olive oil to the boiling water for spaghetti?
When you cook spaghetti, do you add olive oil to the boiling water?
cooking.stackexchange.com
<oil><boiling><olive><spaghetti>
cooking.stackexchange.com/questions/3986
cooking.stackexchange.com/users/1576/Rasmus
It is true that Italians usually don't add olive oil to the water when cooking dried pasta, but they do add a little with fresh homemade pasta. Oil is added to the sauce at the end, when the pasta is on the plate. While adding oil to water is often thought to help the pasta not to stick, this apparently does not work. However, it can help to reduce foaming and the need to stir as much. In general, Italians don't add oil to cooking water for pasta. This is said to prevent the pasta sticking together in the first few minutes. It also stops the water from foaming up.
Italians do add oil to cooking water unless it is for fresh pasta. It does not help with dry pasta sticking together. Adding oil to the water is supposed to stop it sticking together. Oil is an anti-foaming agent and it stops the water from foaming up too much.