Off the top of my head - assuming the OST is accessible via Outlook:
1. Download MDaemon Free (nothing stops you from using a trial version of the full-featured MDaemon if the free
version doesn't support IMAP) and install it on your very PC where you are using Outlook.
2. Create and account on Mdaemon and connect to that account using IMAP4 Protocol
3. Copy mail from the folders on the Outlook (OST) into similar folder names on the IMAP connection to MDaemon
4. Once completed, create a .pst data file on Outlook.
5. Drag the folders from the IMAP to this .pst
6. M-Pesa 910 to my number below.
e&oe!!