because of something obviously true.

Roberta refused to respond to the charge, and with good reason – it was true.

• on the scene
• on the scrap heap
• on the shelf
• on the side
• on the sidelines
• on the side of the angels
• on the sly
• on-the-spot
• on the spot
• on the spur of the moment
• on the starting blocks
• on the strength of
• on the strength of something
• on the stump
• on the table
• on the threshold of
• on the threshold of something
• on the tip of tongue
• on the tip of your tongue
• on the town
• on the trot
• on the verge of
• on the verge of doing something
• on the wagon
• on the warpath