Heat oil in a large pot over medium-high heat. Add bacon, and cook until crisp. Remove bacon from pan, crumble and return to the pan. Add onion, and cook until tender, about 5 minutes. Add garlic, and cook until just fragrant. Add collard greens, and fry until they start to wilt. Pour in chicken broth, and season with salt, pepper, and red pepper flakes. Reduce heat to low, cover, and simmer for 45 minutes, or until greens are tender.