Love Icebreaker
I bought this shirt for my dad because he is often cold and he likes wool but often wears a cotton t-shirt underneath to keep the wool clean and because of itch. (Two problems you don't have with icebreaker! It's very soft and easily washed) I choose this shirt because I have many icebreaker shirts myself and particularly love the "bodyfit200" line both for wearing as a baselayer or for wearing on it's own. He's very happy as well! He normally wears a large in "regular" clothes but he found this large a bit too tight in the arms/shoulders. The extra large did fit well so maybe think about going a size up depending on how fitted you want the shirt.

Great shirt, but pricey
I love this shirt! I'm 5-11/150lbs and the medium is loose enough for me to be comfortable wearing it as an outer layer when it is not too cold. It is a bit looser than other base layers (I like the looseness, but others may not). Note that the sleeves are quite long, and may be annoying for people with shorter arms. This shirt is very comfortable, though like many other reviewers have said, running it through the wash a couple times really helps with the scratchiness. After 2-3 washes, it feels soft as cotton. Warmth is only average but moisture wicking is great, as you would expect from wool. Overall a good buy, IF you're in need of a performance base layer. It's certainly expensive for a shirt, but worth it if you need it.

baselayer or alone
Love this shirt. Excellent value for someone who loves merino. I have some of the more expensive styles in the Icebreaker line as well, but find that the Oasis pretty much gives me the same functionality, although less styling than the GT series. I have about 10 icebreaker shirts in various styles both short and long sleeve. I use most of them for active wear in high endurance running or cycling. I used to use synthetics and completely abandoned them because of their short lifespan (too stinky). I can't remember the last time I wore something other than merino for training. As far as fit, this shirt is on the snug side. I like a next to skin fit to take advantage of the wool's 'tech' properties. In general, I have not found better fitting clothing for active wear than in Icebreaker. I'm 5'9" and 150lbs and usually go with medium, but for the Oasis crew, a size L gives me a little more room and still fits next to the skin. Sleeves and torso will run a little long in Icebreakers (good for my body measurements). (Large in the GT series is slightly too big for me.) Pay attention to fit description in Icebreaker, as some styles are meant to be more form fitting. As for weather conditions: For cycling at about 15-20mph for 2 hours, this shirt alone works well for me in the 45 to 75 degree temperature range. Any colder and I put another layer on top. Any warmer and I go with short sleeve. Color stays amazingly vivid on wool (wash after wash) so the brighter colors work great for visibility. I wash these shirts in a machine at warm/cold with liquid woolite and they come out smelling great. Always hang dry unless you want to shrink it. I can get 2-3 uses in-between by just rinsing with water. Icebreakers are very durable at the 200 weight (150 is a little more prone to wear and tear).

Excellent base layer
I own several Icebreaker products and find them to be excellent for outdoor activities. The wool packs small and regulates body temperature very well, without having the odor problems that synthetics can have. The Oasis 200 can be worn by itself for hiking in 50-60F degree weather, or layered for hiking in colder weather. Wool does a much better job of keeping me at just the right temperature when I'm doing anything aerobic compared to synthetics.

Best Base Layer
Like every other Icebreaker piece I have purchased, it is outstanding. A great base layer during this colder-than-normal winter we're having here in Ohio. I've worn it under a single outer layer of spun polypro and it's been more than warm enough in temps down in the teens, even in brisk winds. It insulates well even when I've worked up a good sweat. And it's absolutely true what they say about merino wool - it simply does not carry the odors I've found in synthetic base layers. I'm 5-10 and 175 with longer than average arms, and the XL fits me perfectly in both the body and sleeve. I'm absolutely hooked on merino wool and especially Icebreaker. Top quality in every way.