The OR Gate: Truth Table and Symbol

The OR gate takes two inputs and produces an output of 1 when at least one input is 1. It is one of the three fundamental logic gates (along with AND and NOT) and is essential knowledge for OCR J277 Section 2.5.

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How the OR Gate Works

The OR gate implements logical disjunction. The output is 1 (true) when input A or input B (or both) is 1. The output is 0 only when both inputs are 0.

In Boolean algebra, OR is written as:

• A OR B
• A + B (a plus sign, but this means OR, not addition)
• A v B (in some notations)

OCR Exam Tip: Be careful not to confuse the Boolean OR symbol (+) with arithmetic addition. In Boolean algebra, 1 + 1 = 1 (not 2). The + symbol means OR, not "add".

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OR Gate Truth Table

A	B	A OR B
0	0	0
0	1	1
1	0	1
1	1	1

The output is 0 in only one row — when both inputs are 0. In all other cases, the output is 1.

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OR Gate Circuit Symbol

The standard OR gate symbol has a curved back (input side) and a pointed front (output side), resembling an arrowhead or shield shape:

  A ----\       /---- Q
        |  OR  |
  B ----/       \

The two inputs enter from the left, and the single output exits from the right. There is no bubble at the output.

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OR in Programming

In Python:

# Using the or keyword
is_weekend = True
is_holiday = False

if is_weekend or is_holiday:
    print("No school today!")
else:
    print("Time for school")

In OCR pseudocode:

isWeekend = true
isHoliday = false

if isWeekend OR isHoliday then
    print("No school today!")
else
    print("Time for school")
endif

Since isWeekend is true, the OR condition evaluates to true — you only need at least one condition to be true.

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Real-World Analogy

Imagine two light switches connected in parallel controlling a single light bulb:

Switch A	Switch B	Light
Off	Off	Off
Off	On	On
On	Off	On
On	On	On

The light turns on if either switch (or both) is on. This is an OR operation.

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Comparing AND and OR