Can anyone advise how to close off the OSS period? My client's VAT period is bi monthly and the OSS period is quarterly?

Hi jeddison-uptonry, thanks for posting on the Community

Please note we don't directly support OSS returns in QuickBooks Online UK. To track transactions relating to this, you may consider setting up a custom tax agency. Here, you can set the period to be quarterly. 

The only separate feature to close off a period in QuickBooks would be the close books feature, which is generally used when closing out the year to prevent unwanted changes.