BECMI to OSE To Hit & THAC0

OSE uses +1 to hit, or THAC0 for monsters. BECMI uses look-up-in-a-table, based on HD. For my own convenience I've laid out the conversion here.

HD	Up to 1	1+ to 2	2+ to 3	3+ to 4	4+ to 5	5+ to 6	6+ to 7	7+ to 8
To Hit	0	+1	+2	+3	+4	+5	+6	+7
THAC0	19	18	17	16	15	14	13	12
HD	8+ to 9	9+	10+	11+	12+	13+	14+	15+
To Hit	+8	+9	+10	+11	+12	+13	+14	+16
THAC0	11	10	9	8	7	6	5	4

... it actually keeps going after that, but it's sufficient for my purposes.

Here's a classic BECMI illustration of demi-humans from the Mentzer Basic set.
(Larry Elmore)