OTMA was an acronym sometimes used by the four daughters of Russian emperor Nicholas II

Nicholas II of Russia

Nicholas II was the last Emperor of Russia, Grand Prince of Finland, and titular King of Poland. His official short title was Nicholas II, Emperor and Autocrat of All the Russias and he is known as Saint Nicholas the Passion-Bearer by the Russian Orthodox Church.Nicholas II ruled from 1894 until...

 and Alexandra Feodorovna

Alexandra Fyodorovna of Hesse

Alix of Hesse and by Rhine later Alexandra Feodorovna Romanova , was Empress consort of Russia as spouse of Nicholas II, the last Emperor of the Russian Empire...

 as a group nickname for themselves, built from the first letter of each girl's name in the order of their births:

• Ольга - Olga Nikolaevna Romanova
  Grand Duchess Olga Nikolaevna of Russia
  Grand Duchess Olga Nikolaevna of Russia ; , November 16 after 1900 – July 17, 1918) was the eldest daughter of the last autocratic ruler of the Russian Empire, Emperor Nicholas II, and of Empress Alexandra of Russia....
  
   (15 November 1895 - 17 July 1918)
• Татьяна - Tatiana Nikolaevna Romanova
  Grand Duchess Tatiana Nikolaevna of Russia
  Grand Duchess Tatiana Nikolaevna of Russia , , was the second daughter of Tsar Nicholas II, the last monarch of Russia, and of Tsarina Alexandra...
  
   (10 June 1897 - 17 July 1918)
• Мария - Maria Nikolaevna Romanova (26 June 1899 - 17 July 1918)
• Анастасия - Anastasia Nikolaevna Romanova
  Grand Duchess Anastasia Nikolaevna of Russia
  Grand Duchess Anastasia Nikolaevna of Russia was the youngest daughter of Tsar Nicholas II of Russia, the last sovereign of Imperial Russia, and his wife Alexandra Fyodorovna....
  
   (18 June 1901 - 17 July 1918).

In childhood the grand duchesses came up with ОТМА as a sign of sibling closeness and affection for one another, writing it in their diaries

Diary

A diary is a record with discrete entries arranged by date reporting on what has happened over the course of a day or other period. A personal diary may include a person's experiences, and/or thoughts or feelings, including comment on current events outside the writer's direct experience. Someone...

. The girls were great granddaughters of Queen Victoria

Victoria of the United Kingdom

Victoria was the monarch of the United Kingdom of Great Britain and Ireland from 20 June 1837 until her death. From 1 May 1876, she used the additional title of Empress of India....

 and, although "thoroughly Russian," grew up speaking both Russian and English fluently among themselves. Whilst the family was in captivity after the Russian Revolution of 1917

Russian Revolution of 1917

The Russian Revolution is the collective term for a series of revolutions in Russia in 1917, which destroyed the Tsarist autocracy and led to the creation of the Soviet Union. The Tsar was deposed and replaced by a provisional government in the first revolution of February 1917...

they were allowed to send few letters so the sisters often signed this nickname on cards they had written together for loved ones and friends.