Only prop Adam Jones was missing as he continues his recovery from the shoulder injury he suffered during the second Test defeat to the Springboks at the end of June.
Having been allowed a longer-than-normal summer break following their exploits with Britain and Ireland's elite, Bowe, Byrne, Alun-Wyn Jones, Phillips and Williams will now begin the same four-week cycle already undertaken by other members of the Ospreys squad.
The returning Lions will initially focus on muscle mass and body composition, as well as fitness training, before moving on to more explosive exercises and speed work in the second half of the programme.
"All the players reporting back have participated in the same four-week programme that we have put together that will see a gradual build up in the amount and intensity of training over three weeks, with the fourth week dropping off a bit, and this group of players will also follow the same schedule," said Ospreys strength and conditioning coach Mark Bennett.
"They will be worked hard but it's controlled and part of a carefully structured programme to get the best results out of them.
"We've put together a series of intense tests that will make sure the players get the right direction and guidance during pre-season rather than simply being worked to death day-in day-out as can be the case.
"The series of tests that we've had the players doing today will highlight any areas of weakness that they have, see where they are exactly and then allow us to focus both their and our energy in the right direction."