Onport is usually configured so that items are routed to be either in-house or drop ship based on the brand name. There are more advanced routing rules available as well.
In some instances, you may need to override just a single SKU. For example, set a specific SKU to be in-house, while all other items under that brand name will continue to be dropship.
To do this, go to Inventory > Choose the product you want to change > Shipping tab. From there, you'll be able to select the preferred routing (e.g. in-house) then the warehouse (for in-house) or dropship provider (for dropship) to send the items to.
This won't impact any orders retrospectively, only new orders being raised by customers. So anything historic may need to be migrated as a second step.