With a clutch of young rivals still in pursuit of the former world champion, the heptathlon finale is set to be a testing one.
The leader is Johnson-Thompson with 4,718 points, 122 ahead of O'Dowda.
Kate O'Connor may be the main threat to Johnson-Thompson.
The competition ends with javelin and 800m.
Johnson-Thompson had an overnight advantage of more than 100 points and would have extended it by more.
The 22-year-old soared out to 6.52m, 18 cm higher than her previous best.
Johnson-Thompson produced a best of 6.33m on her last attempt.
O'Connor is expected to emerge out of the pack this evening, with previous form suggesting that may be as good as it gets for O'Dowda.
Johnson-Thompson is a stronger thrower than the 21-year-old who is competing in the javelin event later in the week.
In June, O'Connor threw 51.20m, which is close to her personal best.
The season's best distance for Johnson-Thompson is 40.78m. O'Connor's advantage is based on the difference in their javelin bests.
Johnson-Thompson ran two minutes 7.26 seconds to win the world title, but she hasn't done anything similar since. At both the World Championships and the multi- event meeting in Gotzis, she had a time of 2:19.
O'Connor dropped her 800m personal best to 2:11.76 last year and was on her way to the Tokyo Olympics when she injured her ankle.
She said before the Commonwealths she would only be content with a place on the podium. The top step could now be part of her plans.
Matthew Hudson- Smith returned from the World Championship with a bronze medal.
After breezing through the 400m heats in 46.26 seconds, the Birchfield harrier pointed to the sky and the track beneath his feet as he celebrated.
He said that his success in Oregon is in the past.
I want three medals and I have said that all year. I'm excited about that and we have a plan for it.
I'm embracing the fact that I have a target on my back. I'm just going to enjoy the sun.
England's Victoria Ohuruogu was the fastest in the women's one-lap heat with a time of 51.34 seconds.
A 20-year-old English 800m prospect won his heat ahead of a defending champion. He will be joined in the final by his team mates Jamie and Guy.
England's Max Burgin, the fastest man in the world this year, may have withdrawn through injury, but Sunday evening's clash still promises a lot.
Peter Bol's story of fleeing South Sudan as a child and his fourth place finish at the Tokyo Olympics made him a hero in Australia.
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