When a Sales Order is marked as signed or approved it is possible to automatically have a linked Opportunity marked as Closed and Won.
From Settings (top right) > Additional Settings > Sales Order - Opportunity Settings block - tick the Update Linked Opportunity status tick box, set the Sales Stage to be your closed stage and the sales order status that will trigger the update, similar to the example shown below;
When the sales order is saved with the specified status, if there is a linked opportunity, the opportunity sales stage will be updated to the value specified.
The "Probability" percentage related to the "Sales Stage" will be set on the Opportunity and the "Weighted Amount" will be recalculated based on the new value.
Note: the stage specified must be an existing sales stage or the probability value will be set to 0 and the weighted amount calculation will fail and be set to 0.