Can I substitute olive oil for butter/shortening in pie crust?

The trick to incorporating olive oil into your crust is to freeze it first until it's opaque and congealed, "like the consistency of slightly melted sorbet." From the recipe for an olive oil double crust; it has a "surprisingly neutral taste... [and by freezing it] helps the fat blend into the dough in little pockets, creating the flakiness you crave (Moskowitz, Vegan Pie in the Sky p. 39)"

Hot-water or raised-pie doughs, I have had success with olive oil.

With your average apple-pie crust (mock or not), the consistency seems to be more fragile in rolling out and shaping: got better results by pressing the dough directly into pan after sprinkling in rather streusel size pieces, if you can picture that.

Advantage there is that the dough need not come completely together first and can be played with in the pan until even thickness whereas kneading folding and re-rolling can easily lead to toughness.

I constantly run across statements that it's not possible to make a flaky oil-based crust. However, this is simply not true.

My grandma's oil/milk based pie crusts are consistently flakier than any butter/shortening/lard based crust I've ever made or tasted.

However, it's important to point out that this bit of wisdom is so common because you cannot simply substitute oil for butter/shortening/lard in a traditional pie crust recipe.

A flaky oil based crust requires a different recipe and careful handling of the dough when mixing and rolling. For those interested, give this a try:

2 C flour + a little salt
5/8 C oil
5/16 C milk 

It's easiest to use a glass measuring cup so you can measure 
out the oil and then *pour the milk into the oil* before 
pouring it into the flour.

**CAREFUL**
Pour the liquid into the flour and *gently* stir the liquid 
and flour together with a fork.  You only want ~12-14 stirs. 
*DO NOT* beat the mixture or knead the dough. 

After this gentle mixing, pack the dough together and divide in half.  
The dough should have a *wet/oily* consistency; it should *not* be dry and crumbly.

Roll the dough out between two sheets of wax paper.
If you mixed things correctly, the dough should have a 
marbled or mottled appearance and should be wet enough 
to patch together easily.

Makes enough for two pie shells (or a top and bottom crust)

If you haven't worked with crusts between wax paper before, here's the trick for transferring it to the pie tin:

Gently remove the top sheet of wax paper and place it lightly back on the crust.
Quickly flip the dough over (grab two corners and flip the crust).
Remove the sheet of wax paper now on the top.
Place your hand under the bottom sheet of wax paper and flip the crust into a pie tin.  
Gently form the dough into the tin and then carefully remove the wax paper.
Cut any excess dough off the edges of the pie tin and use this to patch 
any tears or fill in any gaps.

You can further improve the consistency (i.e., flakiness) of the top crust (or bottom crust if you're cooking the pie shell on its own for a pudding or fresh fruit pie) by rubbing a little milk on the surface of the crust and sprinkling a little sugar (or cinnamon/sugar for things like the top crust of an apple pie).

I've been using this recipe for 20+ years and friends/guests are always surprised that such a flaky/delicate crust was oil based. Others are surprised at how simple and easy it is to make. It's a great all purpose crust that I use with fruit pies, meat pies, pudding/custard pies, etc. The pie crust itself is great on its own (sprinkle some cinnamon and sugar on it after rubbing it with milk and then cook some "pie crust cookies").

I'm obviously biased because this is the crust that my mom and grandmother always made when I was growing up, but I find the consistency and flavor to be better than typical shortening crusts. (Although, to be fair, my mother-in-law makes an amazing lard/vodka crust and, frankly, some recipes call for a more shortbread-like crust).

There is absolutely no problem in making pie crusts with oil instead of butter/shortening.

The important thing to remember is that it's not a 1:1 substitution. In my favourite pie crust recipe, which is my own veganization of the Cook's Illustrated vodka pie crust, the substitution is 65 grams of oil for 100 grams of butter, and about a tablespoon of water.

Normally, I would use "yellow" oil, one of the canola/soybean/rapeseed neutral oils for this sort of thing. Olive oil has a special flavour which I'm not sure I would want in an apple pie. The ratios would be the same, but olive oil has the flavour, and is more expensive, so I tend to avoid it except for savoury pies.

I don't think any sort of oil would work at all for a flaky pâte brisée type of flaky pastry crust. Those kinds of pastry rely on having layers of solid fat separated by dough. As the pastry cooks, the fat melts into the dough but leaves the distinct flakes. Oil might be useful in other types of crust, such as some sort of crumb crust.

A much better vegan substitute for pie crust is coconut oil, for one thing, it tastes like coconut. Honestly I think I like it better than butter for fruit pies. It is also very nice for curried meat pies. For me, olive oil is more heavy and greasy and not how I want a fruit pie to taste. I would also try almond oil, which for me goes well with both sweet and savory. I do think coconut oil is easier to work with in a crust dough. As others have pointed out, it is not a 1 to 1 sub, since butter is a semi-solid emulsion of air, water, and fat and oil is just liquid fat.

I come from a olive oil producing country. Every single person that knows how to cook in my family has the following rule:

Olive oil for savory recipes. Corn/sunflower oil or butter/margarine for sweet ones.

It wasn't until I disobeyed this rule, that I found out why it came to exist in the first place. Let me tell you it wasn't pleasant throwing those doughnuts in the garbage, after all the ingredients and effort I put on them.

Olive oil has a distinctive and strong taste that you don't want to overthrow the other components of your sweets. It can get a lot worse if your olive oil is of poor quality, which in my experience is very possible if you don't live in southern Europe (some of my relatives struggled to find olive oil that tasted 'home-like' when they lived abroad).

So, when you are making sweet pies opt for unsalted butter/other oils with mild flavor.

I agree that oil crust are very flaky, but as others have said, they are delicate. I just tried making a lattice top for the first time, for a mock apple pie (using green tomatoes), and the strips kept tearing. But oil crust work well for double crust pies. You can also refrigerate the dough for the top crust for 10-20 minutes and grate it on a box grater, to make crumble.

I have made pie crust cookies using different oils and done side-by-side taste tests. I like peanut oil best, which has a rich flavour, though avocado oil (a bit fruity) is acceptable for fruit pies. Canola oil tastes fishy; I have stopped using it.

I have used melted butter and water and compared it to peanut oil and water, and peanut oil and 2% milk. As you can imagine the peanut oil and water had the least flavour; it was also the most crumbly. I find the flavour of peanut oil and 2% milk competes well with butter and water. If you use milk as the liquid in an oil crust you still get the non-fat milk solids (NFMS) that make butter taste good. I did the math and in order to get the same amount of NFMS you should dilute the milk by 1/4 to 1/3, but I usually use straight milk...In my experience oil crusts made with 2% milk are sturdier than those made with whole milk. I have not tried using 1% or skim milk.

Note that extra virgin olive oil contains natural mono- and diglycerides, which are emulsifiers. I recently tired using EVOO as the oil in a crust for a quiche; the crust was very soggy. I think the emulsifiers in the EVOO bonded with the liquid in the filling. I won't use EVOO for pies with wet fillings again.