OST file comes with default storage of 20 GB in Outlook 2007 but it is 50 GB in Outlook 2010 and above but comes with 20 GB by default. If OST file reaches its limit then following errors may occur which leads to Oversized OST and ost file also may be corrupt and to overcome from that errors you can fix it manually by disabling Add ins in Outlook or deleting some mails but it takes too much time so you also can take help of OST to PST converter which slit your files in parts so that you can access your file or you can either compress the file to reduce its size.