Petty Officer

Petty officer refers to enlisted naval personnel in grades E-4 and higher. In most common use, it refers specifically to the grades from E-4 to E-6:

Code	Grade Title (and abbrev.)
E-4	Petty Officer Third Class (PO3)
E-5	Petty Officer Second Class (PO2)
E-6	Petty Officer First Class (PO1)

The enlisted grades E-7 and above are more commonly referred to as "chiefs," even though the term "petty officer" is part of their grade titles:

E-7	Chief Petty Officer (CPO)
E-8	Senior Chief Petty Officer (SCPO)
E-9	Master Chief Petty Officer (MCPO)
E-10	Senior Master Chief Petty Officer (SMCPO)

References

TRMN Ranks
Type	Grade	Navy	Marines/Army	Type	Grade	Fleet	Marines/Army
Officers				Enlisted
Flag Officers	F6	AotF HADM GdF	MotC MotA	NCO Ranks	E10	SMCPON OSBdF FSMCPO CSMCPO MCPON SMCPO OSB	SGTMAJRMMC SGTMAJRMA CSGTMAJ RSGTMAJ
	F5	FADM GADM	MSHL FMSHL		E9	MCPO SB	SGTMAJ
	F4	ADM	GEN		E8	SCPO HB	1SGT
	F3	VADM	LTGEN		E7	CPO OB	MSGT
	F2	RADM KADM	MAJGEN		E6	PO1 B	SSGT
	F1	CDRE FADM	BRGEN		E5	PO2 OMT	PSGT
Field Grade Officers	O6	CAPT CAPTSG CAPTJG KDS	COL		E4	PO3 MT	CPL
	O5	CDR FK	LTCOL	Non-Rate Enlisted Ranks	E3	S1C HGEFR	LCPL
	O4	LCDR KK	MAJ		E2	S2C OGEFR	PFC
Company Grade Officers	O3	LTSG KL	CPT		E1	S3C GEFR	PVT
	O2	LTJG OLDS	1LT	Training Ranks	N/A	MID CDT SK
	O1	ENS LDS	2LT
Warrant Officers
W5		W4		W3		W2	W1
MCWO SMCWO		SCWO MCWO		CWO SCWO		WO1 CWO	WO2 WO