Morgl the Oracle is a newly added Shaman hero in Hearthstone, and you get him trough the Recruit a Friend-promotion. In order to unlock him, you need to recruit a friend that earns a total of 20 levels across all the different classes in Hearthstone, so if you have a friend or two that are on the fence to playing the game, now is a good time to persuade them.
However, the easiest way is to get him all by yourself. Since the game is free, you can use your own promotion code to create a new account and level up the classes yourself. Getting to level 20 should only take between 30 minutes and 1 hour, and there is no need to purchase anything at all. The longest part is doing the non-optional introduction quest, but luckily you only need to do this once on the new account.
The fastest way to level up is by playing against yourself with your new account. Keep conceding with your main account, and the new account will get some XP each time! Start by playing and conceding two games with each class, so that they reach level 2. This gives you a total of 18 levels, so the only thing left is to keep playing with your favorite class until it reaches level 4. This is a few more games than to get level 2, but it's done in a few minutes. It can all be done by conceding with your main account, or if you want it the hard way, just play against a few different players, and you will reach level 4 in no time at all!