Dwight Howard has gotten the most career offensive rebounds by a player younger than 21, with 647 rebounds.

nba most offensive rebounds by nba player under age 21

NAME	OREB	GP	MPG	PPG	RPG	APG	SPG	BPG	TPG	FGM	FGA	FG%	3PM	3PA	3P%	FTM	FTA	FT%	MIN	PTS	REB	AST	STL	BLK	TOV	PF	+/-
Dwight Howard	647	184	34.9	14.2	11.5	1.3	0.8	1.6	2.4	5.1	9.5	53.5	0.0	0.0	0.0	4.0	6.4	62.9	6,415	2,619	2,109	233	152	292	449	562	-106
Andre Drummond	609	141	27.4	11.1	10.8	0.5	1.1	1.6	1.2	4.9	7.9	61.8	0.0	0.0	25.0	1.4	3.5	40.2	3,862	1,571	1,528	65	160	226	167	417	-284
Jalen Duren	421	128	26.9	11.4	10.2	1.7	0.6	0.8	1.7	4.7	7.5	63.1	0.0	0.0	0.0	1.9	2.6	70.7	3,448	1,453	1,304	222	77	106	221	374	-590
Tracy McGrady	413	192	24.7	11.1	5.5	2.5	1.0	1.4	1.6	4.2	9.4	44.8	0.2	0.7	28.4	2.4	3.4	71.2	4,747	2,122	1,048	474	191	278	306	381	-262
Andris Biedrins	411	172	21.0	6.5	6.5	0.7	0.6	1.2	0.9	2.9	4.8	60.8	0.0	0.0	0.0	0.7	1.6	46.0	3,606	1,122	1,122	120	97	199	159	551	+131
Isaiah Stewart	385	139	23.5	8.1	7.7	1.0	0.4	1.2	1.1	3.4	6.5	52.9	0.3	0.8	33.0	1.0	1.4	70.8	3,270	1,126	1,070	141	61	164	152	395	-427
Alperen Sengun	380	147	24.9	12.3	7.3	3.2	0.9	0.9	2.3	4.7	9.0	52.2	0.3	1.2	27.6	2.5	3.5	71.3	3,660	1,801	1,071	476	129	138	338	472	-592
Kevin Garnett	365	157	33.7	13.7	7.1	2.4	1.2	1.9	1.8	5.8	11.7	49.6	0.1	0.3	28.6	2.0	2.7	73.6	5,288	2,144	1,119	381	191	294	285	388	+116
Josh Smith	356	169	30.5	10.6	6.6	2.1	0.8	2.3	2.0	3.9	9.1	43.2	0.3	1.1	27.1	2.5	3.5	70.3	5,159	1,799	1,110	363	142	381	338	467	-697
Chris Bosh	354	141	34.8	13.7	8.0	1.4	0.9	1.4	1.8	5.0	10.6	46.5	0.0	0.1	33.3	3.7	5.2	72.2	4,901	1,930	1,124	203	122	194	254	401	-140
Anthony Davis	348	119	31.9	16.8	9.1	1.2	1.3	2.3	1.5	6.5	12.6	51.9	0.0	0.1	15.4	3.7	4.9	76.2	3,802	2,001	1,084	148	153	272	184	330	-242
Jarrett Allen	335	152	23.3	9.6	7.0	1.0	0.5	1.4	1.2	3.7	6.3	59.0	0.1	0.4	18.3	2.0	2.8	73.2	3,537	1,460	1,060	159	71	208	185	331	-264
Derrick Favors	329	143	20.4	7.7	5.9	0.6	0.5	0.9	1.3	3.1	6.0	50.8	0.0	0.0		1.6	2.6	62.4	2,911	1,103	837	82	67	133	180	391	-255
Carmelo Anthony	324	157	35.7	20.9	5.9	2.7	1.1	0.5	3.0	7.4	17.2	42.8	0.7	2.4	29.8	5.5	7.0	78.7	5,603	3,283	924	421	165	71	471	454	+231
Bismack Biyombo	307	143	25.5	5.0	6.6	0.4	0.3	1.8	1.1	2.0	4.4	45.7	0.0	0.0		1.0	2.0	50.2	3,641	713	950	62	48	258	153	340	-833
Joe Smith	300	82	34.4	15.3	8.7	1.0	1.0	1.6	1.7	5.7	12.5	45.8	0.1	0.3	35.7	3.7	4.8	77.3	2,821	1,251	717	79	85	134	138	224
Antoine Walker	288	82	36.2	17.5	9.0	3.2	1.3	0.6	2.8	7.0	16.5	42.5	0.6	1.9	32.7	2.8	4.5	63.1	2,970	1,435	741	262	105	53	230	271	-559
Tyson Chandler	283	146	22.0	7.7	5.9	0.9	0.4	1.4	1.6	2.8	5.4	51.8	0.0	0.0		2.1	3.5	60.6	3,216	1,127	857	130	65	199	234	399	-513
Shareef Abdur-Rahim	281	101	35.2	19.3	7.0	2.2	1.0	0.9	2.9	7.1	15.5	45.6	0.1	0.3	24.1	5.1	6.7	75.4	3,555	1,946	711	224	96	91	289	252	-704
Andrew Bynum	278	163	19.3	7.2	5.6	1.0	0.2	1.4	1.1	2.9	5.0	57.1	0.0	0.0	0.0	1.4	2.2	64.9	3,140	1,171	920	164	28	222	184	403	+80
Elton Brand	276	60	36.0	19.0	9.8	1.8	0.9	1.5	2.9	7.5	15.5	48.1	0.0	0.0	0.0	4.1	6.1	67.0	2,160	1,138	586	110	53	92	171	203	-496
Kevin Love	274	81	25.3	11.1	9.1	1.0	0.4	0.6	1.5	3.9	8.5	45.9	0.0	0.2	10.5	3.3	4.1	78.9	2,048	899	734	84	35	50	118	205	-270
Amar'e Stoudemire	269	91	31.8	13.7	8.9	0.9	0.7	1.1	2.3	4.9	10.4	46.7	0.0	0.1	16.7	4.0	5.9	66.9	2,891	1,249	809	79	67	104	207	301	+20
Eddy Curry	262	171	19.2	9.3	4.4	0.5	0.2	0.8	1.5	3.6	6.7	54.0	0.0	0.0		2.1	3.2	64.5	3,283	1,585	744	81	41	136	255	471	-520
Jabari Smith Jr.	261	155	31.4	13.3	7.6	1.4	0.6	0.9	1.2	4.8	11.1	43.0	1.7	5.0	33.5	2.0	2.5	79.8	4,874	2,054	1,185	222	95	135	193	427	-638