How do I get obsidian in survival?

There are two ways to get obsidian in survival mode:
1. Mix lava and water together;
This will only work if the lava and water are still and not flowing. If you mix them flowing they will simply turn into cobblestone, you may test this in creative if you wish as it works in both modes.
2. Create a Nether Core Reactor and Activate it;
You will need three diamond gems and six iron ingots to make a Nether Core Reactor and four gold blocks and fourteen cobblestone to activate it. The gold, cobblestone and reactor must be placed the right way for this to work, here is the link for a good video to show you:

http://m.youtube.com/watch?v=2TWf0jeI7bA&desktop_uri=%2Fwatch%3Fv%3D2TWf0jeI7bA

After it is activated Items such as bones, seeds and quartz will pop out of the ground and unless your in peaceful mode zombie pig man will apear too. This will last for about one minute.

Be careful not to activate it too close to your house because it will spawn quite a big tower thing (made of nether rack) In the center of this will be a 3x3x3 cube of obsidian (in the center will be your nether core reactor which can be mined with a iron pick) you can mine the obsidian from there (this only works in survival mode).

You have to use diamond pick axe to mine obsidian

I hope this helps