Hide Opportunity Field on Sales Order Form

1. Navigate to Customization > Forms > Transaction Forms.
2. Customize the Standard Sales Order form.
3. Click on Screen Fields tab.

Result: On the Main tab the Show check box of the Opportunity field is grayed out.

To hide the field from the Sales Order form, perform the following steps:

4. Click on Move Elements Between Subtabs button.
5. Mark the Select check box of the Opportunity field.
6. Select a Subtab which is not in use (or hidden).
7. Click Save
8. Make sure that the Show check box of the subtab selected in the Move Transaction Forms Element window is not checked.
10. Save.