You may wish to disable temporarily sleep and hibernate functions on your computer so not to interrupt the files sync half-way.
Method #1 - Using the Web browser
If your files are not too large, you can simply drag the root folders over to your files in OneOffice. The folder and its contents will be recursively uploaded.
Don't close the web page, as this interrupt the upload process
Method #2 - Using OO-Sync [Recommended]
Download and install the Desktop sync app -- here. Chose an easy location for your local sync folder (e.g. [Your Home Directory]/OneOffice)
Wait until it looks stable (at the beginning, there is quite a bit of data being sent back and forth to ensure proper sync between your computer and the cloud)
In Files Explorer, move all the folders and files you want to sync onto the local sync folder created by OO-Sync in step 1