The assassin rule says you can include one AOTI in a Patrol, Battalion and Brigade Detachment. Unfortunately those arent AoO detachments.
AGENT OF THE IMPERIUM
If your army is Battle-forged, you can include 1 AGENT OF THE IMPERIUM unit in each IMPERIUM (excluding FALLEN) Patrol, Battalion and Brigade Detachment in your army without those units taking up slots in those Detachments. The inclusion of an AGENT OF THE IMPERIUM unit does not prevent other units from their Detachment from benefiting from Detachment abilities (e.g. Chapter Tactics, Defenders of Humanity etc.), and it does not prevent other units from your army benefiting from abilities that require every model in your army to have that ability (e.g. Combat Doctrines). An AGENT OF THE IMPERIUM unit included in a Patrol, Battalion or Brigade Detachment in this manner is ignored for any rules that state all units from that Detachment must have at least one Faction keyword in common (e.g. in a matched play game) and when determining your Army Faction.
You can however include an AOTI aux support or patrol detachment as per the battle brothers rule. But, assassins dont have the AOTI faction keyword.
BATTLE BROTHERS
If your Arks of Omen Detachment is an IMPERIUM Detachment, you can include one of the following as an Allied Detachment: one AGENTS OF THE IMPERIUM Auxiliary Support or Patrol Detachment; one IMPERIAL KNIGHTS Super-heavy Auxiliary Detachment (containing one FREEBLADE unit); one VOTANN Patrol Detachment.
Then there is the execution force rule. Sadly, no vanguard detachments in AoO games.
EXECUTION FORCE
If your Warlord has the IMPERIUM keyword (excluding FALLEN), you can include this unit in your army as part of a Vanguard Detachment even if that Detachment contains no HQ units. If you do so, that Detachment’s Command Benefits are changed to ‘None’ and its Restrictions are changed to ‘This Detachment cannot include the same datasheet more than once.’
No more assassins in AoO games, or did i miss something ?