The OQL documentation is here
OQL Operators | Mendix Documentation, please review it.
there is no support for and upper/lower operator.
However there is some good news, the LIKE operator is not case sensitive so..
SELECT ID, FirstName, LastName FROM Sales.Customer WHERE Name LIKE 'MARK%' ( or ‘%mark%’ )
matches a Firstname of “Mark”, “MARK”, “mark” , “MArk” etc…
try it out..
Hope that helps.