Ohio 2020 primary absentee ballot statistics for 69 of 88 counties, all with public facing absentee ballot files (includes largest counties):

Requests: 1,554,837
Returned: 1,434,308
Returned on-time: 1,387,627
Returned late: 46,681

Thanks to @BrianAmos for writing the scraper!

I've said repeatedly rural Republican counties are the most stressed to run mail ballot elections. More evidence:

Cuyahoga, Franklin, and Hamilton:
1.7% of all returned mail ballots were late

In the other 66 counties we have data for:
4.1% of all mail ballots were late

Large counties have staff and resources to manage an unusually large volume of mail ballots, while rural counties do not

Urban areas have good internet, making it easier to download a request form earlier

Mail service tends not to run as quickly in rural areas as urban areas

It appears more Ohio voters in urban areas were able to return their ballots on Election Day, perhaps dropping them off at election offices (per suggestion of @jon_m_rob). 12.2% of the big three counties returned their ballots on Election Day, compared with 8.5% elsewhere...

There is a big difference in the number of Ohio ballots returned one day late:

Cuyahoga, Franklin, and Hamilton: 59,589 ballots on Election Day, 1,699 the next day

Other 66 counties with available data: 76,209 ballots on Election Day, 21,126 the next day

This interesting pattern appears to diverge from what is seen in California, where late arriving ballots tend to break Democratic. The big difference in California is every registered voter is mailed a ballot; whereas in Ohio, voters have to go through a request step

Been digging deeper into the Ohio data and I've traced a large number of one-day-late mail ballots to Greene County. Of the 38K mail ballots received by the elections office, 4.7K arrived late, with 4.3K arriving one day late - that's 12.4% late ballots https://lookup.boe.ohio.gov/vtrapp/greene/avreport.aspx

I found a news report of late absentee ballot requests in Greene, but this issue is over 4K ballots arriving one day late. Probably worth the time of an Ohio reporter to look into what happened https://www.daytondailynews.com/news/local/local-voters-didn-get-requested-ballots-changes-called-for-november/tfn0q4MUbxoHEAVNrXY2nL/