Dic: on (review of its usage as meaning: followed by, immediately after, happening at the same time)

• She waited in her hotel to welcome her children on their arrival from London...
• On reaching Dubai the evacuees are taken straight to Dubai international airport.
• Couples are presented with a bottle of wine on their arrival at the hotel
• All patients are examined on admission to the hospital.
• What was your reaction on seeing him?
• On arriving home I discovered they had gone.
• Please report to reception on arrival.
• There was a letter waiting for him on his return.
• On being made aware of the complaints, he informed the committee.
• They exchanged a firm handshake on parting.
• Report to the reception desk on arrival.

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■ You use on when mentioning an event that was followed by another one.

■ [formal] as soon as someone has done something or as soon as something has happened.

■ immediately after something.

■ [happening immediately] FORMAL: used for saying that an event is followed immediately by another event or that the two events happen at the same time.

.... COBUILD, LDOCE, OALD, Macmillan