The Straits Times Index finished up 2.06 points, or 0.10 percent, at 1,919.10, off a three-week high of 1,933.78 hit around midday.
Dealers said profit-taking had weighed on the index after the midday break.
But the profit-taking gathered pace as soon as news broke of rise in oil prices in response to attacks in Iraq on pipelines linking a main southern oil field to pumping stations.
The violence in Iraq pushed up US crude oil more than 50 cents in Asian trade of New York Mercantile Exchange futures.
In the broader market, gainers led losers 169 to 164 in brisk turnover of 637 million shares, although down from Wednesday's 831 million.
Singapore Telecommunication ended about half a percent up at S$2.23, off an early high of S$2.25.
Gains came after its Australian unit Optus Ltd and the Australian unit of British-based Vodafone early on Thursday said they planned to roll out a shared 3G mobile network. Analysts said the move should result in cost savings for the two operators.
Shares of Singapore Airlines ended steady at S$11.00, after hitting a one-month high of S$11.20.
The world's second-largest airline by market value said late on Wednesday it had signed an agreement to purchase 31 Boeing Co long-range 777-300ERs worth about US $7.35 billion in a fleet renewal programme.
UOB, Singapore's third-largest lender, also ended steady at S$13.30 off a two-week high of S$13.40 hit early on.
OCBC finished up 0.76 percent at S$13.30, off an early high of S$13.40, but was helped by news that US investment bank J.P. Morgan has placed the bank as its top pick.
J.P. Morgan maintained its overweight call on Singapore banks following their second-quarter results.
TPV Technology ended up almost three percent at S$1.08 but was down from a three-week high of S$1.11.
The world's No 2 maker of computer monitors, on Wednesday announced its first-half profit which had doubled from a year earlier and said earnings in the second half of the year would match that, despite oversupply and falling LCD panel prices.