One of my end clients has their own timesheet/invoicing portals. Can my worker use their online portal rather than Oncore's?

Yes, an external timesheet system can be used, during the submission of a new contractor the timesheet system would be selected as external/paperbased. When the contractor submits a timesheet through a system that's external to Oncore's, they or the Recruitment Agency would need to send a copy (screenshot; print to PDF; photo) of the approved timesheet to supportaus@oncoreservices.com to be entered into the system.