Olynyk is a quite underrated in a number of areas.
People really only credit him for his ability to shoot jumpers and for his passing ability, but he's much more then that.
Firstly, Olynyk is actually a much better inside finisher then he's given credit for. In every one of his three seasons he's taken at least 35% of his FGA inside three feet, and he's never shot less then 65% on those attempts. Take a look at other NBA players out there and you'll see that those are extremely good numbers.
The reason is that (much like Paul Pierce) he isn't the quickest guy out there, but he does a fantastic job of reading how the defense is playing him and reacting accordingly. He's got very good footwork, good ball handling skills and outstanding basketball IQ.
He'll bait the defense by setting up for a shot, and when they run at him (which they almost always have to, thanks to his impressive shooting ability) he'll put it on the floor, drive hard to the basket, and use their own momentum against them. By doing this he actually gets in to the paint pretty frequently.
When he does get in the paint, he's a very tough cover. He's a MUCH more crafty scorer around the basket then given credit for, so if you let him in the paint he'll get an easy layup.
If the defense doesn't step up and challenge him, he can pull up for the little midrange jumper which (at his height) is very tough to defend. If the defense over-helps and tries to trap him, then he is arguably at his most dangerous - at his height and with his court vision, he'll consistently find the open man.
This is why when Kelly drives, good things almost always happen. Either he gets a jumper, he gets a layup, he kicks it out to an open three point shooter, or he finds an open main in the paint once the interior defenders slide over.
There have been times where in the space of 8-10 game minutes KO has managed only 2 points, and 2 assists, and yet he's had a hand in just about every single basket the Celtics scored over that stretch. When you actually stop and really watch him play - the things he does when he's NOT scoring the ball - you can see how much of an impact he has on the team's offense. It's huge.
Defense is another area where he isn't given nearly enough credit. Again he reminds me a lot of Pierce on D in that Pierce was never a great athlete and never had elite length or lateral quickness, but somehow he always managed to make things hard on opposing wings defensively. He did that by using his mind and out-thinking the opponents. Olynyk is a master at that.
Watch KO play defense and you'll notice how high his defensive IQ really is. He uses angles exceptionally well to force guys into awkward positions, force guys towards shot blockers, etc. He'll get right into an opponent's face and force them to shoot over his (not inconsiderable) height, often leading to missed shots. He's got a quick eye and active hands - knocks away a lot of loose balls, gets a surprising number of steals.
Most importantly he's almost always aware of what's going on. When there are missed rotations and a ball is about to get passed out to an open perimeter shooter, he'll start running out to the shooter before the ball has even left the passer's hand. By the time the shooter catches the ball and starts to get into a shooting motion, there's a big 7-foot man with fuzzy hair flying their way. He is very good at reading what the offense is going to do and reacting as/before they do it.
Another thing KO is really good at on defense - taking charges. He moves better laterally then he's given credit for, and he's really good at picking when a defender is gong to drive, sliding over, and taking the hit.
Even his shot-blocking is better then advertised - his past two seasons he's averaged up around a block per 36. Not exactly a scary rim protector there, but he does affect some shots around the basket from time to time.
His rebounding is a weakness unquestionably (at least it was the past two seasons), but KO other then that KO is a much more "all round" player then people credit him for. He's got to be one of the highest BBIQ bigs in the league, and he makes very good use of it. Very fundamentally sound player.
He is a few ounces of swagger short of being a really, really good player.