OS
Worth 2 points before any premium square, and the lowest-scoring of the 96 — tied with 35 others.
OS extends into 9 three-letter words, which is the 43rd least extendable of 96 — tied with 8 others. That is the number worth memorising: a two-letter word is a place to put tiles, but a well-hooked one is a place to put them where the board stays open.
Every hook
- B→BOS
- C→COS
- D→DOS
- K→KOS
- M→MOS
- N→NOS
- S→SOS
- W→WOS
- E→OSE
Not all of them are equally reachable, which a bare count of 9 hides. OS hooks on 9 distinct letters, and the ones you are likeliest to be holding are E (60% of opening racks), N (36% of opening racks) and D (26% of opening racks). The rarest is K, at 7%.
How often you can actually play it
Out of an opening rack, 10.47% of the time you hold the letters for OS outright, the 34th likeliest of 96 — tied with 4 others. Counting a blank as whichever letter is missing lifts that to 17.07%, though spending one of the two blanks on a two-letter word is rarely the play. Exact hypergeometrics over the 100-tile bag, not a simulation.
It reverses
OS backwards is SO, which is also good. That is how most of this list is actually learned — 46 of the 96 come in reversible pairs, so knowing one gets you the other free.