You shouldn't have to do anything other than export the OST files to Office 365 using Outlook's "Export" function. You could also just create a new PST file and drop and drag the OST files you want to convert. Easy peasy.
Careful though - there are companies that try to trick people into buying unneeded software (OST to PST "converters"). This is almost always rip-off that you don't need, so if you see someone hawking that, it's likely someone dishonest trying to take your money.