What appeared to be a tricky task was actually very simple!
All you need to do it to enter the following code into the item element field.
onchange="this.value = this.value.toUpperCase();"
With this code, the user can type into the field in upper or lower case but as soon as they leave the field the content will be converted to capitals / upper case.
|How to Set Field to Uppercase in Oracle Apex|
The screen shot below is taken from an Oracle Apex 4.2 installation so may look different in other versions.