Babe Ruth has the highest on-base percentage with 100+ at-bats in a month, with an on-base percentage of .611 in July 1920.

Interpreted as:

highest obp by a player in a month with 100 or more at bats

	NAME	OBP	AB	MONTH	G	R	H	2B	3B	HR	RBI	BB	HBP	SO	SB	CS	PA	TB	XBH	SH	SF	IBB	GIDP	AVG	SLG	OPS
1	Babe Ruth B. Ruth	.611	100	July 1920	34	45	44	10	2	13	33	43	1	19	5		147	97	25	3				.440	.970	1.581
2	Ted Williams T. Williams	.592	107	August 1941	35	31	43	4	2	10	26	50	0	8	0		157	81	16	0			2	.402	.757	1.349
3	Charlie Gehringer C. Gehringer	.586	122	August 1937	32	38	59	11	0	7	38	30	0	2	6		152	91	18	0				.484	.746	1.331
4	Rogers Hornsby R. Hornsby	.584	100	June 1925	28	31	48	9	2	9	31	25	0	6	2		126	88	20	1				.480	.880	1.464
5	Lou Gehrig L. Gehrig	.582	111	June 1930	28	37	55	11	3	10	41	20	3	10	5		137	102	24	3				.495	.919	1.501
6	Wade Boggs W. Boggs	.581	101	June 1987	26	26	49	8	4	4	19	22	1	5	0	0	125	77	16	1	0	8	4	.485	.762	1.343
7	Babe Ruth B. Ruth	.580	100	June 1920	29	38	45	7	4	12	37	30	1	17	6		131	96	23	0				.450	.960	1.540
8	George Sisler G. Sisler	.578	114	June 1920	29	33	60	9	3	4	26	13	1	2	9		131	87	16	3				.526	.763	1.341
9	Babe Ruth B. Ruth	.571	111	July 1923	31	36	51	9	3	10	27	29	0	18	1		140	96	22	0				.459	.865	1.436
10	Rogers Hornsby R. Hornsby	.570	106	August 1924	29	29	54	13	2	8	22	15	0	7	0		127	95	23	6				.509	.896	1.466
11	Joe Jackson J. Jackson	.568	102	June 1912	29	28	54	16	8	1		7	2		4		113	89	25	2				.529	.873	1.440
12	Wade Boggs W. Boggs	.567	102	May 1986	27	24	48	9	0	3	20	24	0	5	0	1	127	66	12	0	1	0	2	.471	.647	1.214
13	Ty Cobb T. Cobb	.562	127	July 1912	30	33	67	11	8	3	21	9	1		15		137	103	22	0				.528	.811	1.373
14	Ted Williams T. Williams	.559	104	July 1954	33	31	40	7	0	11	29	45	0	9	0	0	152	80	18	0	3		2	.385	.769	1.328
15	Roy Sievers R. Sievers	.558	103	July 1960	28	31	46	5	0	12	30	25	1	8	1	0	129	87	17	0	0	4	1	.447	.845	1.403
16	Rogers Hornsby R. Hornsby	.556	112	July 1924	31	34	52	8	3	7	27	23	0	6	1		139	87	18	4				.464	.777	1.332
17	Babe Ruth B. Ruth	.556	131	July 1924	37	43	59	9	3	14	40	31	0	16	2		164	116	26	2				.450	.885	1.441
18	Lance Berkman L. Berkman	.553	104	May 2008	28	31	49	11	1	9	22	18	1	18	6	1	123	89	21	0	0	6	1	.471	.856	1.409
19	Mickey Mantle M. Mantle	.551	105	June 1957	30	30	44	7	2	11	30	31	0	17	3	0	136	88	20	0	0	5	0	.419	.838	1.390
20	Babe Ruth B. Ruth	.549	111	July 1927	31	34	47	8	3	9	35	31	0	14	1		145	88	20	3				.423	.793	1.342
21	Todd Helton T. Helton	.548	105	August 2000	29	22	50	18	0	7	32	17	2	7	2	0	126	89	25	0	2	5	1	.476	.848	1.395
22	Harry Heilmann H. Heilmann	.546	104	May 1925	29	21	45	9	4	3	31	26	0	7	1		137	71	16	7				.433	.683	1.229
23	Ted Williams T. Williams	.546	116	August 1949	31	31	47	7	2	10	34	36	0	6	0		152	88	19	0			3	.405	.759	1.305
24	Charlie Keller C. Keller	.543	119	August 1939	30	34	50	4	2	4	27	32	0	13	2		152	70	10	1			1	.420	.588	1.131
25	Ted Williams T. Williams	.540	101	May 1941	26	29	44	7	0	6	22	22	1	3	1		124	69	13	0			3	.436	.683	1.223