According to tradition, Osith was daughter of Frithuwold, the Mercian sub-King of Surrey. Her mother was Wilburga, daughter of King Penda of Mercia. The parents of Osith, with St. Erconwald, founded the monastery of Chertsey (Surrey) in AD 675. She was born at Quarendon, near Aylesbury (Buckinghamshire), and her childhood was spent the care of her maternal aunts, the two holy abbesses, St. Edith of Aylesbury and St. Edburga of Bicester.

There is an old story that St. Edith sent Osith, one day, to take a book to St. Modwenna at her nunnery, in order to point out to her a particularly interesting passage she had discovered. To reach Modwenna's house, Osith had to cross a stream by a bridge. The stream swollen, the wind was high, she was blown into the water and remained there for two days before she was discovered. Edith thought she was safe with Modwenna who, not expecting her visit, was not surprised at her non-appearance. On the third day, Edith, wondering that her pupil had not returned with an answer to her message, came to Modwenna. Great was the consternation of the abbesses when they found they had lost their charge. They went to search for her. Following the banks of the stream, they saw the child lying at the bottom, holding the book open at the passage she had been told to show to Modwenna. The abbesses prayed for her restoration, and commanded her to arise from the water and come to them. This she did: she, her dress and the book being quite uninjured. There is some confusion over which Modwenna is meant here. The story seems to indicate St. Modwenna of Burton-on-Trent, but this is impossible. The lady in question is probably the little known St. Modwenna of Northumbria.

After the death of St. Edith, Osith returned to her parents, who soon accepted, on her behalf, an offer of marriage from Sighere, King of Essex, who reigned jointly with Sebba. Sighere had relapsed into heathenism, but promised to become a Christian on marrying Osith. Osith's inclinations, however, had turned towards a religious life and she would rather have been an abbess than a queen, having secretly made a vow of celibacy. Her fate was decided for her though and she was given to Sighere, whilst still praying that she might have no husband but the Lord. On her marriage, she went with her husband, probably to London, which was then the capital of Essex. On some pretext or other, she declined, for several days, to receive the King in her bower - a separate house for herself and her attendant ladies within the enclosure of the Royal residence. At last, however, her contrivances were exhausted and so was the King's patience. Her seclusion came to a sudden end and her husband stood before her. Still she prayed that she might keep her vow, but Sighere began to protest that, without her, life held no happiness and no interest for him. But even while he spoke, there was a sound of eager voices and hurrying feet. Some of his lords cried, "The stag, the stag" and close to the gate was the largest stag that ever was seen. Up sprang Sighere and, with all his Court, started in pursuit. Osith regarded this interruption as an answer to her prayers and took his departure as a release from her engagement. She sent in all haste for Bishops Acca and Bedwin and, when the King returned, after a chase of four or five days, he found her a veiled nun. He generously gave her an estate at Chich (St. Osiths) in Essex, and built her a church and a monastery, where she soon gathered many holy nuns about to wonderful sanctity.

After many years, on 7th October around AD 700, the Danes made a raid on the Essex coast. Their leader tried by threats and entreaties to make Osith renounce her religion, but in vain and, incensed at his failure, he cut off her head. As it fell to the earth, a fountain bubbled up which, for many years afterwards, had a wonderful power of curing diseases. Osith rose to her feet and carried her head in her hands to the church, staining the door with blood as she opened it. Her family claimed her body and it was buried for a while at Aylesbury Abbey; but the saint intimated, by visions and other signs, that she chose to rest in her own monastery. There, accordingly, she was eventually placed in a rich shrine by Maurice, Bishop of London.

She is represented in art with a stag behind her and a long key hanging from her girdle, or otherwise carrying a key and a sword crossed, a device which commemorates St. Peter, St. Paul and St. Andrew.