Why did he steal the heart Jack worked on if he could already use the Heartless?

I don't think he has enough darkness nor a heart unless you count the hearts of bugs in him.

Maybe he couldn't summon them like Maleficent and the other villains could and it pissed him off, making him try to steal the artificial heart?

 

Or the writers just didn't think about it. KH1 was really light on the story end of things compared to later games- after that one cutscene where all the villains are talking to each other in Hollow Bastion, they rarely interact with each other, especially Oogie and Ursula who just seem to completely ditch Maleficent altogether- and it's likely they just wanted some reason to do things in Halloween Town, regardless of it making sense with the rest of the plot. (You'll notice that nothing that happens in Halloween Town has anything to do with the main story.)

It kind of ties into the strange way that the denizens of Halloween Town see the world. Both Jack and Oogie see the heart in terms of the one thing they know: how to be scared. But while Jack sees such a thing as something that is, in it's own way, good, Oogie would see the fear in the heart in malicious terms, thinking mostly of how he can use that power to raise mayhem. As such, if you had a much more powerful heart, you would have more control over fear, and thus could control the Heartless even more, being as they are manifestations of that fear, in a way.

This. ^^^ If I remember correctly, he wanted the Heart for his own selfish, non-Heartless related reason.

that may be but he may want to give them to the malificent