Otho belonged to an ancient and noble Etruscan family settled at Ferentinum in Etruria. He appears first as one of the most reckless and extravagant of the young nobles who surrounded Nero. This friendship was brought to an end in 58 because of a woman. Poppea Sabina had been taken from her husband by Nero, in order to make her his mistress. Decency required her to be married and so the emperor gave her to his favourite Otho, convinced that there would come no trouble from this husband. But Otho fell in love with her and, when the time came, refused to send her to Nero. After threats and pleads of the emperor, the marriage was annulled and Otho sent as governor to the remote province of Lusitania.
Otho remained in Lusitania for the next ten years, administrating the province with moderation unusual at the time. When in 68 his neighbour Galba, the governor of Hispania Tarraconensis, rose in revolt against Nero, Otho accompanied him to Rome. Resentment at the treatment he had received from Nero may have impelled him to this course, but to this motive was added before long that of personal ambition. Galba was childless and far advanced in years, and Otho, encouraged by the predictions of astrologers, aspired to succeed him. But in January 69 his hopes were dissipated by Galba's formal adoption of Lucius Calpurnius Piso, a man picked randomly in the morning audiences.
Nothing remained for Otho but to strike a bold blow. Desperate as was the state of his finances, thanks to his previous extravagance, he found money to purchase the services of some twenty three soldiers of the Pretorian guard. On the morning of January 15, only five days after the adoption of Piso, Otho attended as usual to pay his respects to the emperor, and then hastily excusing himself on the score of private business hurried from the Palatine to meet his accomplices. He was then escorted to the pretorian camp, where, after a few moments of surprise and indecision, he was saluted imperator.
With an imposing force he returned to the Forum, and at the foot of the Capitol encountered Galba, who, alarmed by vague rumours of treachery, was making his way through a dense crowd of wondering citizens towards the barracks of the guard. The cohort on duty at the Palatine, which had accompanied the emperor, instantly deserted him. Galba, is newly adopted son Piso and others were brutally murdered by the pretorians. The brief struggle over, Otho returned in triumph to the camp, and on the same day was duly invested by the senators with the name of Augustus, the tribunician power and the other dignities belonging to the principate. Otho had owed his success to the resentment felt by the pretorian guards and the rest of the army at Galba's refusal to pay the promised gold to the ones who supported his accession to the throne. The population of the city was also unhappy with Galba and cherished the memory of Nero. Otho's first acts as emperor showed that he was not unmindful of the facts.
He accepted, or appeared to accept, the cognomen of Nero conferred upon him by the shouts of the populace, whom his comparative youth and the effeminacy of his appearance reminded of their lost favourite. Nero's statues were again set up, his freedmen and household officers reinstalled, and the intended completion of the Golden House announced. At the same time the fears of the more sober and respectable citizens were allayed by Otho's liberal professions of his intention to govern equitably, and by his judicious clemency towards Marius Celsus, consul-designate, a devoted adherent of Galba.
But any further development of Otho's policy was checked by the news that reached Rome shortly after his accession, that the army in Germany had declared for Vitellius, the commander of the legions on the lower Rhine, and was already advancing upon Italy. After a vain attempt to conciliate Vitellius by the offer of a share in the empire, Otho, with unexpected vigour, prepared for war. From the remoter provinces, which had acquiesced in his accession, little help was to be expected; but the legions of Dalmatia, Pannonia and Moesia were eager in his cause, the pretorian cohorts were in themselves a formidable force and an efficient fleet gave him the mastery of the Italian seas.
The fleet was at once despatched to secure Liguria, and on the March 14 Otho, undismayed by omens and prodigies, started northwards at the head of his troops in the hopes of preventing the entry of the Vitellius' troops into Italy. But for this he was too late, and all that could be done was to throw troops into Placentia and hold the line of the Po. Otho's advanced guard successfully defended Placentia against Alienus Caecina, and compelled that general to fall back on Cremona. But the arrival of Fabius Valens altered the aspect of affairs.
The Vitellius' commanders now resolved to bring on a decisive battle, and their designs were assisted by the divided and irresolute counsels which prevailed in Otho's camp. The more experienced officers urged the importance of avoiding a battle, until at least the legions from Dalmatia had arrived. But the rashness of the emperor's brother Titianus and of Proculus, prefect of the pretorian guards, added to Otho's feverish impatience, overruled all opposition, and an immediate advance was decided upon, Otho himself remaining behind with a considerable reserve force at Brixellum, on the southern bank of the Po. When this decision was taken, Otho's army had already crossed the Po and were encamped at Bedriacum (or Betriacum), a small village on the Via Postumia, and on the route by which the legions from Dalmatia would naturally arrive.
Leaving a strong detachment to hold the camp at Bedriacum, the Othonian forces advanced along the Via Postumia in the direction of Cremona. At a short distance from that city they unexpectedly encountered the Vitellian troops. The Othonians, though taken at a disadvantage, fought desperately, but were finally forced to fall back in disorder upon their camp at Bedriacum. Thither on the next day the victorious Vitellians followed them, but only to come to terms at once with their disheartened enemy, and to be welcomed into the camp as friends.
More unexpected still was the effect produced at Brixellum by the news of the battle. Otho was still in command of a formidable force: the Dalmatian legions had already reached Aquileia and the spirit of his soldiers and their officers was unbroken. But he was resolved to accept the verdict of the battle that his own impatience had hastened. In a dignified speech he bade farewell to those about him, and then retiring to rest slept soundly for some hours. Early in the morning he stabbed himself to the heart with a dagger, which he had concealed under his pillow, and died as his attendants entered the tent. His funeral was celebrated at once, as he had wished, and not a few of his soldiers followed their master's example by killing themselves at his pyre. A plain tomb was erected in his honour at Brixellum, with the simple inscription Diis Manibus Marci Othonis.
At the time of his death (April 16, 69) he was in his thirty-eighth year, and had reigned just three months.