Theme: "Tic Tac Toe" — Theme answers are familiar phrases that begin with the letter O and end with the letter X.
- 18A: Rival of Staples (OFFICE MAX).
- 25A: Red Cloud's tribe (OGLALA SIOUX).
- 43A: Jane, to Dick, e.g. (OPPOSITE SEX).
- 57A: Pencil game that hints at this puzzle's theme, found in the first and last letters of 18-, 25- and 43-Across (TIC TAC TOE).
- 1A: Roaring group (CROWD). I stuck pride in here with no hesitation.
- 24A: Rides roughshod over (STEPS ON). When I was looking through the puzzle again, I read this is step-son.
- 37A: Make one of two? (UNITE). Not sure I understand the question mark here. Anyone?
- 1D: "Believe" singer (CHER).
- 10D: Nest noise (CHEEP). Tried chirp at first.
- 12D: South American plain (LLANO). This word always reminds me of Sammy Llana of the BoDeans.
- 29D: Jones of English architecture (INIGO).
- 47D: Paper measure (QUIRE). "A quire of paper is today used as a measure of paper quantity. The usual meaning is a set of 24 or 25 sheets of paper of the same size and quality. It might also be thought of as 1/20 of a ream." [wikipedia]
Everything Else — 6A: Bridge coup (SLAM); 10A: Boston NBAer (CELT); 14A: Act like a doting grandma, perhaps (HOVER); 15A: Hawaiian port (HILO); 16A: Healthy (HALE); 17A: Musical narrated by Che (EVITA); 20A: '40s-'50s paranoia that led to blacklisting (RED SCARE); 22A: "For Your Eyes Only" singer Easton (SHEENA); 23A: Island strings (UKE); 30A: Board with eerie messages (OUIJA); 31A: Neither's partner (NOR); 32A: Microwaves (ZAPS); 36A: Not guilty, for example (PLEA); 39A: Came down to earth (ALIT); 40A: Cowpoke's prod (SPUR); 41A: Petunia, e.g. (PIG); 42A: Serpentine (SNAKY); 46A: Fillies and foals (EQUINES); 50A: __ la la (TRA); 51A: Track athlete (RUNNER); 52A: "The Breakfast Club" actors are part of it (BRAT PACK); 59A: Conservative front? (ULTRA-); 60A: Perry's creator (ERLE); 61A: "Major" constellation (URSA); 62A: Grand __ National Park (TETON); 63A: Futurist (SEER); 64A: Seamstress's fold (TUCK); 65A: Not approximate (EXACT); 2D: Wander (ROVE); 3D: "Ars amatoria" poet (OVID); 4D: Antiprohibitionists (WETS); 5D: Count with a cape (DRACULA); 6D: Coastlines (SHORES); 7D: Family board game (LIFE); 8D: TV E.T. (ALF); 9D: Defogging target (MOISTURE); 11D: Chair maker Charles (EAMES); 13D: Houston pro since 2002 (TEXAN); 19D: Whole grain cereal brand (CHEX); 21D: Dossier letters (AKA); 24D: Flue filth (SOOT); 25D: "How clumsy of me!" ("OOPS!"); 26D: Big swig (GULP); 27D: Stead (LIEU); 28D: Slightly open (AJAR); 32D: Author Grey (ZANE); 33D: Quaint word of regret (ALAS); 34D: High-speed highway (PIKE); 35D: Underworld river (STYX); 37D: Pugilist's punch (UPPER CUT); 38D: Puppy bites (NIPS); 42D: Legislative act (STATUTE); 43D: Military service designation (ONE-A); 44D: Windex target (STREAK); 45D: Lyricist Gershwin (IRA); 46D: Art works by Romain de Tirtoff (ERTÉS); 48D: "I surrender!" ("UNCLE!"); 49D: Bury (INTER); 52D: Pear choice (BOSC); 53D: Suffix with cine (-PLEX); 54D: "__ boy!" (ATTA); 55D: Gator's kin (CROC); 56D: "Critique of Pure Reason" philosopher (KANT); 58D: Capote, on the stage (TRU).