I'll be happy to share with you some information about converting to QBO to ensure you are able to convert the file correctly.
Yes, you can use a QuickBooks version older than 2018, but, it can't be any older than a 2015 version. In order to convert a QuickBooks Desktop company file, it must come from a supported version of QuickBooks.
Here at Intuit, QuickBooks versions older than 3 years are discontinued every May 31st of the current year. You can check out this article for more information about the discontinuation:
Since we haven't reached this date yet, that means, you can still convert your QuickBooks Pro 2015 company file without using a trial version. It will only be necessary to use the trial version if you converted this file after May 31.
Now that we've cleared that up, let's go ahead and convert your file to QBO. We can refer to this help article for the steps to convert the file:
Let me know how it goes. I'll be here if you need more help converting to QuickBooks Online.