634-42. Oswald of Northumbria overlord of the Mercians (?)
There is no absolutely clear evidence for this overlordship of Oswald over the Mercians. It was postulated by Nicholas Brooks to explain how Bede and the Chronicle can assume that Penda's reign starts in 626 or 633 (see entry on c.626), whereas the Historia Brittonum assumes it begins after the battle of Maserfelth in 642, and that at that battle, Eowa, king of the Mercians, was slain, and Penda separated Mercia from the Northumbrians. Bede's remark that Penda ruled "with varying success" from 633 suggests that the 642 date might represent Penda regaining his kingdom, rather than gaining it for the first time.
Penda and Cadwallon were allies in the Battle of Hatfield of 633, in which they killed Edwin and devastated Northumbria. Bede's account of the Battle of Heavenfield in the following year notes only the crushing defeat of Cadwallon. Nothing is said of Penda's fate, but it may be that the Northumbrians supplanted him and installed Eowa as a subking of Mercia, under the overall authority of Oswald of Northumbria. This would then be the situation until the battle of Maserfelth in 642, when Penda regained power and defeated Eowa and so freed Mercia from the Northumbrians, as the Historia Brittonum explains.
This sequence would closely parallel the documented Northumbrian overlordship over Mercia in 655-8, when after the Battle of Winwæd in which Penda was killed, Oswiu sent Northumbrian ealdormen to govern Mercia, which they did for three years until the Mercians supplanted them with Penda's son Wulfhere. It may be that the Mercians suffered the yoke of Eowa longer because he was himself a Mercian, the brother of Penda (both are called sons of Pybba: see ASC 626, 757).
N. Brooks, "The formation of the Mercian kingdom", in S. Bassett (ed.), The Origins of the Anglo-Saxon Kingdoms (London: 1989), pp.159-70, at pp.166-8