Condition: jumps if OF = 0. After a signed ADD, SUB, INC, DEC, NEG or IMUL it means the result is exactly right; the "happy path" of checked arithmetic usually continues with JNO OK1 while the fall-through handles the error. After CMP it means the subtraction did not overflow, which on its own says nothing about which value is larger.

Prints Y. Change 20 to 30: 130 overflows and it prints N.
MOV     AL, 100
ADD     AL, 20   ; 120 still fits a signed byte: OF = 0
MOV     DL, "Y"  ; MOV leaves the flags alone
JNO     SHOW1    ; taken: DL stays "Y"
MOV     DL, "N"
SHOW1:
MOV     AH, 02h  ; DOS: print the character in DL
INT     21h

Mind the difference with the carry: MOV AL, 200 / ADD AL, 100 gives OF = 0 (as signed it is −56 + 100 = 44, which fits) but CF = 1, since the unsigned sum 300 does not fit. JNO would happily accept it. Choose OF for signed data and CF for unsigned data.

Range

On the 8086 JNO exists only in the SHORT form: two bytes, opcode 71h and a signed 8-bit displacement counted from the end of the instruction. The target must lie within −128..+127 bytes of that point. To go farther, invert the test and hop over an unconditional JMP, which can reach the whole segment. MASM 6 makes this rewrite itself when it finds the target out of range (its default OPTION LJMP), so the listing may show two instructions where you wrote one.

JO      SKIP1    ; the opposite test, a short hop
JMP     FARAWAY  ; any distance in the segment
SKIP1:

Later CPUs

The 80386 added a near form of every conditional jump, 0Fh 81h followed by a 16-bit displacement (32-bit in 32-bit code), so from the 386 on JNO can reach anywhere in the segment. Code for the 8086 must stay with the short form.