This starts Word and prevents add-ins and global templates from being loaded automatically. This article, we can also open word with /a command switch. Unselect all add ins to test the results. In addition to safe mode, I'd like to recommend you manually unselect add ins in Word to test the results. So, when you Open Word in safe mode, it will not lock the file as add ins are all disabled.
Usually this is the result of a certain add-in that you have installed or an application that relies on the outlook.exe process. Would you mind pasting whole error message here for further research? If you try to rename or delete the OST file, you will receive a statement that the file is currently locked by Word or Excel respectively.
