I had an old OST file that wasn’t just full of contacts, but also had some key information I needed in vCard format so I could import it into my CRM software. II tried doing it manually, but the data either came out incomplete or the format would mess up. After a lot of time wasted, I found
GainTools OST to vCard Converter. Honestly, I didn't think it would work very well, but it did. It let me save only the contacts from the OST file as separate VCF files, so I didn't lose any information. So even though my case wasn’t the usual “just need contacts from OST,” this tool still got the job done perfectly. If you're stuck with something similar, you should try this.