I'm only using OpenERP version 7, and since I'm new, I'm not sure if it will be always easy but I have tested exporting from and importing back to OpenERP, and they work fine.

First, make sure to check [✓] the option Allow users to import data from CSV files under

Settings Module -> Configuration -> General Settings -> Import/Export

Now, you can go to the list view of your [old company] products or customers, check [✓] those you would like to copy to the new company, click the [More] button (near print button at the top) and click Export. There will be a lot of field names that you may include, but you can just double click the field names that you want, on the left pane, so that they will included during the export. Just leave all the other default settings and click [Export to File] to download your .csv (comma separate values),

[optional] If you want, you can edit product.product.csv or res.partner.csv in Microsoft Excel and edit them. Last time I used LibreOffice calc, it butchered my CSV file, so I'm sticking with excel for now.

You can now import the .csv file from the old company to your new company, using the same products or partner/customer list view. You can validate your csv file data before you Import and you will even see a preview of the data before you insert or update to the company database.