I used OpenOffice for many years but I've switched to LibreOffice, which is a spin-off from OpenOffice that's more up-to-date. (For example, while OpenOffice can read Word files that have been created in the newer .docx format, it can only save them in the older .doc format. LibreOffice can both read and write files using the latest Microsoft formats).
https://www.libreoffice.org/
[Note: There's absolutely no problem with having BOTH OpenOffice and LibreOffice on your PC at the same time. I've got those two, as SoftMaker Free Office as well, on my own computer].
To answer your question though (and irrespective of which of the free office suites you choose to use), your new software will be able to open your existing Microsoft Word files directly (from File > Open), without the need for you to do anything.
However it's possible that your computer might retain the existing 'file association', which tells it to try to use Microsoft Word when your double-click on an existing file. To get around that, right-click on a random Word file and select 'Open with'. Then select 'Choose another app' (even if the one you want to use is already shown in the list). Then choose the correct app and put a tick in the box marked 'Always use this app to open .docx' files before clicking 'OK'. You'll only need to do it once for Word files but if you've got Excel files, or similar, you'll need to repeat it for each file type.