I see some opportunity line items are deleted in my org but not sure who deleted it. I tried Export all in data loader to get the deleted ones but I couldn't get it. Is there any other way to fetch?
Generally Opportunity Line Items will not store in recycle bin when it's deleted by user, still you want to retrieve the data you have to contact salesforce support but they use to charge for this.
To prevent this, you have to write trigger logic on OLI object and get the record information before delete and store to some other custom object / send an email to admins with the deleted information / restrict user to delete the OLI.
Please let me know if you required any more information on this.