Let's take it step by step :
$OU = (("CN=SPARE-LAPTOP,OU=Laptops,OU=Workstations,DC=domain,DC=local").TrimEnd(",DC=domain,DC=local") -split ",")
This gives the following array of strings :
CN=SPARE-LAPTOP
OU=Laptops
OU=Workstations
Now, let's filter the items starting with "OU=" and get rid of the "OU=" in these items :
$OUMatches = $ou | Select-String -Pattern "^OU=" | ForEach-Object { $_ -replace "OU=", "" }
Then, reverse the order in the array :
$Reversed = $OUMatches[-1], $OUMatches[0]
This gives us the following value for $Reversed :
Workstations
Laptops
Now, we just need to transform our array back to a string :
$Reversed -join '>'
This ouputs the following :
Workstations>Laptops