=(COUNTIF($M$1:$M$4999,"OK")=ROWS($M$1:$M$4999))+0

Custom format the formula cell as:

[=0]"No";[=1]"Yes"

Steved wrote:
Hello from Steved

$M$1:$M$4999

if M1:M4999 all has "OK" the answer is yes

if M1:M4999 all has not got "OK" the answer is No

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