The burly Trescothick hit 219 in a nine-and-a-half-hour innings with 32 fours and two sixes while Thorpe, in the wilderness for a year, compiled a clinical 124 as England closed on 502 for seven, a narrow first innings lead of 18.
The South Africans, who are leading the series 2-1, fought back well towards the end of the day to get rid of Alec Stewart, Trescothick and Ashley Giles for 22 runs in the closing overs.
England's hopes of a substantial lead now rest entirely on the powerful shoulders of Lancashire all-rounder Andrew Flintoff, who remained not out on 10 with Martin Bicknell for company.
The home team, having dismissed Graeme Smith's dominant touring side for 484 on a good Oval track, resumed on 165 for two and dominated most of the day to keep their slender hopes of salvaging the series alive.
The two left-handers combined perfectly to put on 268 for the third wicket before Jacques Kallis bowled Thorpe off his pads with an off-cutter just before tea.
The Surrey player, his career stalled by a bitter marriage break-up that led to him quitting all cricket for a period last year, batted for five-and-a-quarter hours for his 12th Test century containing 17 fours.
Thorpe was one of three England players to get a standing ovation on the day. "To score a hundred on your return, and on your home ground, I couldn't have written it better," he said. "It was probably harder than my debut [Thorpe had scored a century on debut as well] because I desperately wanted to do well.
"There was a lot of emotion and relief -- I felt like I climbed a mountain.
"There are no regrets. I have had to play the cards dealt to me. I feel like my career has been put back on track."
Thorpe, consistently overlooked before getting another chance after former captain Nasser Hussain broke a toe in the fourth Test, is now in line to make England's winter touring squad.
Ed Smith, one of the men selected ahead of the 34-year-old Thorpe, failed again before Trescothick and Stewart, given an ovation in his final Test and applauded to the wicket by his opponents, put on 101 for the fifth wicket.
South Africa are seeking their first series win in England since 1965.
Thorpe and Trescothick will probably value their contrasting performances higher than any previous ones.
Thorpe's century was a masterclass in guile as he kept rotating the strike with clever singles while unleashing drives and elegant cuts.
His partner, best known for his full-blooded strokeplay as well as his vulnerability outside the off stump, seemed to learn from Thorpe's example as he tightened his technique.
The pair added 106 in the morning without alarm before being parted on the stroke of tea.
Trescothick, tiring during the final session, seemed to lose his concentration and was almost caught by a diving Paul Adams at third man off Makhaya Ntini as he passed his previous Test best of 161.
There was time for a pulled six off spinner Adams before Trescothick fell to a hook off Ntini with a handful of overs remaining, Jacques Rudolph taking the catch at fine leg.
England (1st innings):
M Trescothick c Rudolph b Ntini 219
M Vaughan c Gibbs b Pollock 23
M Butcher lbw Hall 32
G Thorpe b Kallis 124
Ed Smith lbw Hall 16
A Stewart lbw Pollock 38
A Flintoff not out 10
A Giles c Hall b Kallis 2
M Bicknell not out 0
Extras: (b-10, w-3, nb-6, lb-19) 38
Total: (140 overs, for 7 wickets ) 502
To bat: S Harmison, J Anderson
Bowling: S Pollock 33-8-103-2; M Ntini 27-4-97-1; A Hall 30-5-101-2; J Kallis 29-4-90-2; P Adams 15-2-63-0; J Rudolph 6-1-28-0
