Former captain Cook battled patiently on to 72 after Vernon Philander, who had spent 50 minutes off the field with stomach trouble, had taken his second wicket of a rain-interrupted day, the prized one of Cook’s successor as skipper, Joe Root.
With the four-Test series locked at 1-1, the visitors took the initiative after lunch with Cook, still unperturbed after 161 balls at the crease, and Philander, who had excellent figures of 2-8 from nine overs, the key protagonists.
Root had elected to bat on winning the toss, giving Cook the chance to impress once again in the landmark Test at the south London venue which has become one of cricket’s most historic grounds over 137 years.
The Oval joined another London venue Lord’s and Australia’s Melbourne and Sydney Cricket Grounds in hosting a century of Tests.
In a new-look England side, featuring three new caps, batsmen Westley and Dawid Malan and seamer Toby Roland-Jones, it was old stager Cook who summoned all his resilience and skill to anchor the innings in conditions favouring the Proteas’ pacemen.
In an awkward morning session in overcast conditions and with a green tinge to the pitch, South Africa’s attack did not initially pose quite the menace they had offered in their 340-run victory in the previous Test at Trent Bridge, which had levelled the four-match series.
Cook was given capable support by Essex county colleague Westley in a half-century stand before lunch but the newcomer departed for 25 straight after the resumption, pushing hard at Chris Morris to edge to second slip.
Westley had impressed after Keaton Jennings was again found wanting, prodding tentatively at Philander to be caught at third slip for a duck in the fourth over.
Cook, on 28, survived an lbw review, the faintest of inside edges off Morris sparing him, but otherwise looked his usual model of serenity as he made it to an early lunch, after two rain stoppages, on 34 with England at 62 for one.
The South Africans imposed more pressure on the resumption with Philander dismissing the dangerous-looking Root for 29, thanks to a dazzling, diving one-handed catch from wicket-keeper Quinton de Kock.
Kagiso Rabada, back in the team after suspension that saw him miss the second Test, sent debutant Malan packing for one with a lovely inswinging yorker.
Still, though, Cook ploughed on after another rain break, maintaining his focus to bring up his half-century off 128 balls with a handsome cut for four off Philander.