Hi,
If you are having an OST file that quite large in size, then I would like to suggest a software that I used to resolve the same issue. You can make use of OST Recovery Tool. This software converts your OST file into PST and while making the conversion, it provides you with option of splitting your PST file into number of files as per size. You can explore more features and get this OST Recovery Tool by going to the following link: -http://www.exchange.ostrecovery.co.uk/ .
Regards