There is very strong support for the idea that the Cretaceous flightless creatures such as the Ornithomimosauria and the Oviraptorosauria were secondarily flightless (and not dinosaurs at all):
"Dromaeosaurs have been thought to be ground-dwelling
dinosaurs that represented ancestral stages of how birds
evolved. Fossils in this exhibit show that they have been
misinterpreted as dinosaurs when they are actually birds.
Feather impressions reveal that they had flight feathers on the
wings and a second set on the hind legs. Even without the
feathers preserved, the avian characteristics of the skeleton
demonstrate that these dromaeosaurs are birds. This discovery
means that the larger dromaeosaurs, like Deinonychus and
Velociraptor of “Jurassic Park” fame, were really feathered and are secondarily flightless birds."
"Martin believes that maniraptorans are secondarily flightless birds, and that birds evolved from non–dinosaurian archosaurs [pterosaurs, according to what I am saying], so that most of the species formerly called theropods would now not even be classified as dinosaurs."
"On the contrary, unmentioned by them is that abundant paleontological evidence has led several workers to conclude that troodontids and oviraptorids were secondary flightless birds. This evidence ranges from bird-like bodies and bone designs, adapted for climbing, perching, gliding, and ultimately flight, to relatively large, highly developed brains, poor sense of smell, and their feeding habits. Because ratites also are secondarily flightless and tinamous are reluctant, clumsy fliers, the new evidence strengthens the view that troodontids and oviraptorids were secondarily flightless." (Kavanau JL.)
"A hypothesis, credited to Gregory Paul and propounded in his books Predatory Dinosaurs of the World (1988) and Dinosaurs of the Air (2002), suggests that some groups of non-flying carnivorous dinosaurs, especially deinonychosaurs but perhaps others such as oviraptorosaurs, therizinosaurs, alvarezsaurids and ornithomimosaurs, are actually descended from birds. Paul also proposed that the bird ancestor of these groups was more advanced in its flight adaptations than Archaeopteryx. This would mean that Archaeopteryx is thus less closely related to extant birds than these dinosaurs are.
Paul's hypothesis received additional support when Mayr et al. (2005) analyzed a new, tenth specimen of Archaeopteryx, and concluded that Archaeopteryx was the sister clade to the Deinonychosauria, but that the more advanced bird Confuciusornis was within the Dromaeosauridae. This result supports Paul's hypothesis, suggesting that the Deinonychosauria and the Troodontidae are part of Aves, the bird lineage proper, and secondarily flightless. This paper, however, excluded all other birds and thus did not sample their character distributions. The paper was criticized by Corfe and Butler (2006) who found the authors could not support their conclusions statistically. Mayr et al. agreed that the statistical support was weak, but added that it is also weak for the alternative scenarios."
"Oviraptorosaurs ("egg thief lizards") are a group of feathered maniraptoran dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot - like skulls, with or without bony crests atop the head.
Analyses like those of Osmolska et al. (2004) suggest that they may in fact represent primitive flightless birds."