Even though some restaurants in Dotonbori opens for 24 hours, it is best to go here at night. Dotonbori area is livelier at night with all the LED lights and stuff. 

You don't have to worry about getting lost in Dotonbori and the street is easy to navigate. When you see Starbucks Coffee (or Tsutaya in Japanese), you are standing at the main area of Dotonbori.

While I was at Tsutaya, I decided to get for myself a tumblr because Osaka's tumblr looks awesome. 

Right across Tsutaya, you will see a restaurant with a huge crab plastered on its billboard. The restaurant has a super long queue on its entrance, it kinda makes me think twice whether to join the queue or to explore other vendors. It turns out to be the most popular crab restaurant in Dotonbori,  Dotombori Honten. 

Dotombori Honten offers a variety of crab dishes and Kani Suki (crab hot pot). The problem is, it is nearly impossible to get a table without any reservation if you only reached Dotonbori around 6PM-ish. Knowing the fact that the restaurant is very popular, Dotombori Honten also offers a take-away shop in front of the restaurant with equally long queue. The queue was worth it though because the grilled crabs are very very delicious.   

I suggest you should get some food items from the other restaurants while your friend is queueing for the crab so that both of you can munch on something while waiting for your turn at Dotombori Honten. This can only work if you are not a solo-traveller, I suppose.