Shinnosuke Oka extended Japan’s winning streak to four Olympics when he claimed the men’s gymnastics all-around gold medal on Wednesday, but admitted the standard of the competition was not high. The Asian world champion, 20, showed few nerves on his Games debut to tally 86.832 points from the floor, pommel horse, rings, vault, parallel bars and horizontal bar. In a tight contest in Paris, Japan edged China yet again having snatched gold in Monday’s team event thanks in part to Oka. China’s 2021 world champion Zhang Boheng took individual silver after flopping early on the floor, but he fought…