From the Home Screen, touch and hold a widget or an empty area until the apps jiggle.
Tap the Add button + in the upper-left corner.
Search and select 'Opal Travel' widget, then tap Add Widget.
Tap and hold the Opal Travel' app widget to choose the 'saved trip' you want.
Select one of the 'Saved trip'.
Tap Done.

Tips:

You can add another widget and choose a different 'Saved trip' to your Home Screen by following the same steps above.
If you don't have a saved trip to choose from, just setup and save a new trip using 'Departures' option. To save a new trip for widget, tap 'New trip' then 'Departures' from the bottom navigation menu and follow the prompts. Learn more about How to save a trip for OTA departures widget.
You can switch and scroll through multiple Opal Travel widgets by stacking up together. Learn more via How to stack widgets on iPhone - Apple Support

Opal Travel departure board widget

How to add and edit the Opal Travel app widget on your Apple iPhone home screen