O in Morse Code: ---

History of Letter O in Morse Code

The letter O in Morse code is --- — three dashes, and nothing else. It is the only three-signal letter composed entirely of dashes, and its deep, sustained sound is one of the most instantly recognisable patterns in the International Morse alphabet. O was standardised in the global system adopted by the International Telecommunication Union (ITU) in 1865, which brought a single coherent alphabet to telegraph networks worldwide.

Three dashes places O in a category shared by no other letter — it is the pure-dash equivalent of S (...), which uses three pure dots. Together O and S define the extremes of the three-signal tier: one is three slow, weighty tones, the other three fast, light taps. At any operating speed, --- arrives as a slow, rolling wave — operators often compare its sound to a ship's foghorn giving three equal blasts — deep, evenly spaced, and impossible to mistake.

O is the fourth most frequent letter in English text, yet its three-dash code is longer than the two-signal codes of less common letters like A, I, M, and N. This reflects the ITU code's historical compromise: O's current representation replaced an earlier, shorter version and settled into the all-dash triple that became the global standard.

Morse Code Alphabet Chart — Letter O in Context

Every letter in International Morse Code uses between one and four signals. O (---) is the only three-signal letter composed entirely of dashes. See where it sits among the full alphabet:

O (---) and S (...) are the purest three-signal contrast in the entire alphabet — three slow heavy dashes versus three fast light dots. Building them into a repeating loop --- ... --- ... trains your ear on the most fundamental tonal opposition at the three-signal level. This pair also forms the skeleton of SOS (... --- ...), giving every Morse learner an immediate real-world context to anchor both letters from their very first session.

Tips for Memorising Letter O in Morse Code

Three dashes — O is the deepest, most sustained letter in Morse code. Here are four techniques to make --- completely automatic:

• Build the dash ladder: T is one dash (-), M is two (--), O is three (---). Practise - -- --- as a rising three-step sequence, feeling each letter grow heavier by one unit. This drill locks in T, M, and O simultaneously — the three pure-dash letters — and builds an intuitive understanding of how dash quantity defines this family of letters. After mastering the ladder, recognising O is simply a matter of counting three dashes.
• The "DAH-DAH-DAH" chant: Say "DAH-DAH-DAH" with three equal, deliberate beats. There is no internal variation — no dots, no timing shifts, no asymmetry. O is Morse code at its most uniform. That uniformity makes it one of the easiest letters to send cleanly, because the only challenge is sustaining three identical heavy beats without rushing or shortening any of them.
• Contrast with S (...): S is three dots and O is three dashes — same number of signals, opposite weight. Drill ... --- ... --- in a loop. The contrast is total: fast and light vs slow and heavy, repeated in alternation. This is the foundational three-signal contrast drill in Morse code, and it provides the structural skeleton of SOS as a free bonus — two letters and one iconic phrase in a single exercise.
• Use SOS as your anchor: Every Morse learner knows SOS. Send it at the start of every practice session as a warm-up — ... --- ... — three dots, three dashes, three dots. After a few weeks of this warm-up ritual, --- will fire automatically the moment you encounter O anywhere, without any conscious recall. The world's most famous distress signal becomes your most effective single-letter mnemonic.

Practice: What Is the Morse Code for O?

Select the correct Morse code for O:

How to Remember Letter O in Morse Code

Related Morse Code Letters

Other letters often learned alongside O: