Gloomy overhead conditions in south London, allied to a green-tinged pitch freshened by early morning rain threatened to make life tough for the batsmen.
"We went with three seamers but now we're going with four so hopefully the workload will be shared," said De Silva, celebrating his 33rd birthday on Friday, at the toss.
England stand-in captain Ollie Pope, in charge for the whole series after Ben Stokes was ruled out with a torn hamstring, added: "We would've bowled first too, looks like a bowl first but a great opportunity for us to challenge ourselves with the bat."
Top-order batsman Pope, who plays at the Oval for Surrey, has managed just 30 runs in four innings since taking over the captaincy.
"Runs haven't been easy to come through but another opportunity to go out and get runs on the board," he said.
Both teams made changes from last week's second Test at Lord's, where England thrashed Sri Lanka by 190 runs -- their seventh win in a row against the islanders -- to take an unassailable 2-0 lead in a three-match series.
England gave a debut to 20-year-old left-arm fast bowler Josh Hull, in place of Matthew Potts.
The 6 feet, 7 inches (2 metres) tall Hull was included despite having taken just 16 wickets in 10 first-class matches.
Hull, however, took five wickets for the second-string England Lions against Sri Lanka in the tourists' lone warm-up match last month.
Sri Lanka made two changes, Kusal Mendis returning to the top order after Nishan Madushka was dropped and left-arm paceman Asitha Fernando recalled in place of left-arm spinner Prabath Jayasuriya as the visitors opted for a four-man seam attack.
England, who whitewashed the West Indies 3-0 earlier in the season, are bidding for their first home Test campaign clean sweep since 2004 when Michael Vaughan oversaw seven wins from seven.
Victory for either side at the Oval would also boost their hopes of qualifying for next year's World Test Championship final at Lord's, with England currently fifth in the standings and Sri Lanka in seventh place.
Teams
England: Dan Lawrence, Ben Duckett, Ollie Pope (capt), Joe Root, Harry Brook, Jamie Smith (wkt), Chris Woakes, Gus Atkinson, Olly Stone, Josh Hull, Shoaib Bashir
Sri Lanka: Dimuth Karunaratne, Pathum Nissanka, Kusal Mendis, Angelo Mathews, Dinesh Chandimal (wkt), Dhananjaya de Silva (capt), Kamindu Mendis, Milan Rathnayake, Lahiru Kumara, Vishwa Fernando, Asitha Fernando
Umpires: Joel Wilson (WIS), Chris Gaffaney (NZL)
TV umpire: Paul Reiffel (AUS)
Match referee: David Boon (AUS)
