Numsgil wrote:
11 is a mistake. It worked out okay in game, but it shouldn't have. Having gotten this far you need to play K17.
Here's the thing:
1. If you want more claim on the top (which is what you say when you play O16), you need to finish it off with a move at K17 or thereabouts. If you don't play here, and white approaches your corner, you pretty much have to respond for very few points, and white can lightly jump back and claim most of the top. That is, you're leaving a sente play for white. One that's big enough that white could probably take it immediately (if you don't respond, white can scoop out your base, and now you have a weak group on the run).
2. If you want sente to approach one of white's 3-4 stones, play O16 at O17 (3rd line). That gives you stability and you can take sente. Alternatively, you can just ignore white's approach to your 4-4 and approach one of his 3-4 stones.
3. You shouldn't reasonably expect to get sente out of approaching the bottom left corner. Generally speaking, the approach-er to a corner loses sente. The fact that you did get sente feels like white made a mistake.
I might have to disagree. Even if White approaches the top right with a move like M17, it is very easy to move out to the center because O16 is high. Furthermore, Black still has S17 to make base if he needs to if White gets strong in the area (which he isn't currently). For instance:

Gu Li 9p (B) vs. Yamashita Keigo 9p (W)
- Click Here To Show Diagram Code
[go]$$Bcm1 Gu Li 9p (B) vs. Yamashita Keigo 9p (W)
$$ -----------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . X . . . . |
$$ | . . . O . . . . . , . . . . . , X . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . 6 . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . a , . . . . . , . . . . . , O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . 2 . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . X . . . |
$$ | . . 4 . . 1 . . 5 . . . . . . . . . . |
$$ | . . . 3 . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ----------------------------------------[/go]
Yamashita prioritizes a two-space extension on the right over 'a', even though White can always make an extension by miai there.
Another example:

Liu Xing 9p (B) vs. Fang Jie 7p (W)
- Click Here To Show Diagram Code
[go]$$Bcm1 Liu Xing 9p (B) vs. Fang Jie 7p (W)
$$ -----------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . 2 . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . 1 . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . b . . . . . . . . . . . . . . . . |
$$ | . . b , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . a . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . 6 . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . 4 . . . . . , . . . . . , 3 . . |
$$ | . . 8 . . 5 . . 9 . . . . . 0 . . . . |
$$ | . . . 7 . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ----------------------------------------[/go]
In this case, a corner approach is prioritized. And in this game, a move around 'a' for B or 'b' for W was never made.

Shao Weigang 6p (B) vs. Yang Hui 8p (W)
- Click Here To Show Diagram Code
[go]$$Wcm11 Shao Weigang 6p (B) vs. Yang Hui 8p (W)
$$ -----------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . O . . . . . . . . . . . . . |
$$ | . . . X . . . . . , . . . . . X . . . |
$$ | . . 1 . . . . . . . . . . . . . . . . |
$$ | . . . X . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . X . O . O . . . , . . . . . O . . . |
$$ | . . O . . . . . . a . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ----------------------------------------[/go]
And here, again an extension at 'a' for White looks fine, but instead White chooses to..well, I'm not sure but yeah.
And just to make myself clear, I am not saying it's bad to make an extension. In fact, I think it's simpler and perfectly fine in almost all cases, including the game the OP posted. However, I also think there are times when it's not unreasonable to tenuki, even when the board has very few stones.