There should be an oil content monitor fitted to the cascade tank on an UMS ship, this should be regularly tested and if it is not working the ship should not go UMS but be operated manned, with regular checks of the cascade tank and any other systems where the alarms and automation are not working correctly.
You do not say if this is an auxiliary steam plant or for main propulsion, but either way oil in the boiler can cause localised overheating and boiler tubes to burst, extremely dangerous, so the Boiler should be shut down or run at lowest possible power and pressure as soon as the oil contamination is discovered. Depending on the overall safety of the ship.
Minimum No of people should remain in the Boiler/ Engine Room in case of a Boiler explosion.
There should be a skimmimg connection on the cascade tank to skim off oil from above the water this should be utilised to drain off all the free oil from the cascade tank.
The source of the oil contamination should be found and isolated.
As soon as it is possible to shut down the Steam plant any ordinary detergent can be circulated through the Boiler and feed and condensate system to remove any free oil. The Hot well filters should be replaced.
Any oil that has gone into the boiler is likely to have been carbonised, and will have to be removed with Carbon Remover or by manual cleaning.
Something like this will be a Major Insurance claim and you should involve Superintendents, Class, the Boiler Makers and the your Chemical supplier for detailed Technical advice, and support, all as soon as possible. Expect to be laid up in Port for a while while all this is done and a full Boiler Survey carried out to ensure that all the oil has been removed and no damage done, (Hopefully) if not, to decide the extent of repairs required and to carry them out, followed by pressure testing etc.
I hope that this is an exam question and I would warn that I do not have a steam ticket so I would consult with others first, before using my response as the basis for answering an exam question. If this is happening to you now then GOD LUCK and keep safe.
It is always better to ask a stupid question than to do a stupid thing.