"Mary's Boy Child / Oh My Lord" is a 1978 Christmas single for Boney M., a cover of Harry Belafonte's 1956 hit, put in medley with the new song "Oh My Lord" (Farian / Jay).

The single was recorded in a hurry early November, included in the group's live set and rushed out at the end of the month, topping the UK Singles Chart for four weeks and became Christmas number one, spending eight weeks in the charts. It was the second single for the group in the UK's all-time best selling singles list. The song was later included in the group's Christmas Album, 1981.

In the United States, the track reached number 85 in the Billboard Hot 100, Boney M's last of four singles to chart; despite its chart position, the medley is an airplay favorite in the United States during the Thanksgiving/Christmas holiday season on radio stations which change to temporary all-Christmas music formats. It reached number one on the UK Singles Chart and has sold 1.89 million as of November 2015.

A promotional gatefold single (15 816 AT) backed with "Never Change Lovers in the Middle of the Night" (from Nightflight to Venus) was released shortly before the commercial single which had a new track on the B-side "Dancing in the Streets". Different edits were issued in various countries. In Spain and France, the third verse (Now Joseph and his wife Mary came to Bethlehem that night ...) was excised, while the French mix of "Oh My Lord" was a different mix. A 4:29 edit surfaced on the 1980 compilation The Magic of Boney M. – 20 Golden Hits, possibly the originally intended single edit. The full-length 6:18 mix appeared as the B-side of the 12" single "Dancing in the Streets".

Two music videos were produced to promote the single. One featured the band in a white room wearing white furry coats, while the second featured the band once again dressed in white with a nativity scene.

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