As reported here a few days ago, nominations for the contest to become Labour's candidate for Mayor will close next month.
The final date for entrants is the 18th June with the shortlist being finalised on the 24th.
A series of hustings will then take place across the capital.
After the hustings have finished, an electoral college of London party members and affiliated organisations will choose their candidate.
The result will then be announced at Labour's conference in September.
Speaking to pupils at her former school in Camden, Oona is expected to focus on knife crime and it's effect on young people.
In a tone which mirrors that of Boris Johnson's 2008 campaign she will say:
"We must show an interest in them when they are still young enough to be guided in the right direction."Because they aren't the only ones to suffer; damaged human beings leave a trail of victims in their wake, especially as they turn from toddler to teenager and tragically exchange a baby's rattle for a large kitchen knife"
Former Mayor Ken Livingstone is also expected to declare his own candidacy within the next week.
The Liberal Democrats have yet to decide how their candidate will be chosen.
The party will meet soon to discuss whether to choose somebody on the current London Assembly list or to seek out a "celebrity" candidate instead.
Boris Johnson has also not declared whether he will stand again.
I will have more from Oona King's launch later on today. You can follow me on Twitter for the latest.