Question: I have a trigger that updates the value of a custom field on Opportunity objects. I need to set to read-only this field after this uptade. Is there any function to change this property value for a specific field? Thanks

Answer: 
If you want to do this from Apex, I think your best bet would be to modify the trigger so that on update, it only allows the field to be modified IF there is no current value. Something like:

```
trigger OpportunityTrigger on Opportunity (before update) {

  for (Opportunity o : Trigger.new) {
    if (o.field != null && Trigger.oldMap.get(o.id).field != null) {
      o.addError('Field not updatable');
    }      
  }

}

```
Assuming you mean set the particular Opportunities field to read-only, and not this field for all Opportunities. What I would recommend is that you create a field locked checkbox on the Opportunity, and when your trigger updates the record it sets the checkbox to true. Then, you add a validation rule that won't allow the Opp to be saved with a change to that field when the box is checked. 

Your other option, depending on which edition you have, would be to create different record types, with different page layouts. Then, you can change the record type to your "read only" page layout, which has that field set to read only.