I want to visit one of the following cities in August (when the weather in my city is hot and humid) but the more I look into it the more I'm not sure I won't be getting hot weather there as well. For those of you who have experience with these cities, what is the weather generally like in August, and for weather purposes only, do you think one or two has better (not as hot) weather in August? Thanks.

Kansas City
Minneapolis/St. Paul
Oklahoma City
Omaha
Cleveland or Columbus

Given that M/SP is further north, they have to be cooler on average than south in OK where you'll routinely get days in the mid to high 90s
You can look at Aug 2021 daily temps on weather.com. There is more variability day to day than there is north to south, or east to west.

Summer in the Twin Cities would be my choice with all the lakes and better weather.

The average high in August in Minneapolis is 80 F. But it can have high humidity. Itasca State Park/Bemidji just 3.5 hour north is a lot cooler. That's home to the headwaters of the Mississippi and is of national park quality.

Except for MSP, the other cities are likely to have hot weather on random dates in August. MSP is iffy, probably won't be above 85 and humid but it can happen some days, but then the heat will break. Looking for cool weather in Oklahoma in August must be folly.

MSP is a much larger metro area than the others on your list. Aside from the lakes and the Mississippi river gorge and waterfalls which offer biking and boating opportunities, there are 2 excellent art museums. The theater offerings are also excellent, it's a theater town unsupported by tourists for the most part, but August is off season. Some things randomly chosen:

https://www.latteda.org/next-festival-2022

https://www.guthrietheater.org/shows...2-season/emma/

https://www.jungletheater.org/

https://www.parksquaretheatre.org/bo...es-and-watson/

https://artistrymn.org/shrek

MSP and Cleveland aren't that different, pick one of those. Columbus is a bit farther south and not on the lake, of course, so can be a bit warmer than Cleveland but not as bad as the other places you name which can be really hot. OK City is the worst.

I love Kansas City, great food, theater, museum, but Summer can feel like a convection oven. You hope there is no breeze because the breeze can be hot.

Go west and "high" Denver. It will be bright blue skies and maybe high temp but dry dry dry. Lots to do.

I'm originally from Omaha, and have spent time in all of those cities in August. I'd probably agree that Minneapolis is the least bad temperature wise. But as noted, higher humidity and lots of mosquitoes.
Weather is a big reason I moved to Mexico.