The Collaborative International Dictionary of English v.0.48:

Oyer \O"yer\, n. [Anglo F., a hearing, from OF. o["i]r, F.
   ou["i]r, to hear, L. audire. See Audible.] (Law)
   A hearing or an inspection, as of a deed, bond, etc., as when
   a defendant in court prays oyer of a writing. --Blackstone.
   [1913 Webster]

   Oyer and terminer (Law), a term used in England in
      commissions directed to judges of assize about to hold
      court, directing them to hear and determine cases brought
      before them. In the U.S. the phrase is used to designate
      certain criminal courts.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

OYER, pleading. Oyer is a French word signifying to hear; in pleading it is
a prayer or petition to the court, that the party may hear read to him the
deed, &c., stated in the pleadings of the opposite party, and which deed is
by intendment of law in court, when it is pleaded with a profert.
     2. The origin of this form of pleading, we are told, is that the
generality of defendants, in ancient times, were themselves incapable of
reading. 3 Bl. Com. 299.
     3. Oyer is, in some cases demandable of right, and in others it is not.
It may be demanded of any speciality or other written instrument, as bonds
of all sorts, deeds poll, indentures, letters testamentary, and of
administration, and the like, of which a profert in curiam is necessarily
made by the adverse party. But if the party be not bound to plead the
specialty or instrument with a profert, and he pleads it with one, it is but
surplusage, and the court will not compel him to give oyer of it. 1 Salk.
497. Oyer is not now demandable of the writ, and if it be demanded, the
plaintiff may proceed as if no such demand were made. Dougl. 227; 3 B. & P.
398; 1 B.& P. 646, n. b. Nor is oyer demandable of a record, yet if a
judgment or other record be pleaded in its own court, the party pleading it
must give a notice in writing of the term and number roll whereon such
judgment or matter of record is entered or filed in default of which the
plea is not to be received. Tidd's Pr. 529.
     4. To deny over when it ought to be granted is error; and in such case
the party making the claim, should move the court to have it entered on
record, which is in the nature of a plea, and the plaintiff may counterplead
the right of oyer, or strike out the rest of the pleading, following the
oyer, and demur; 1 Saund. 9 b, n. 1; Bac. Abr. Pleas, 1; upon which the
judgment of the court is either that the defendant have oyer, or that he
answer without it. Id. ibid.; 2 Lev. 142; 6 Mod. 28. On the latter judgment,
the defendant may bring a writ of error, for to deny oyer when it ought to
be granted, is error, but not e converso. Id. ibid.; 1 Blackf. R. 126. See,
in general, 1 Saund. 9, n. 1; 289, in. 2; 2. Saund. 9, n. 12, 13; 46, n. 7;
366, n. 1; 405, n. 1; 410, n. 2; Tidd's  Pr. 8 ed. 635 to 638, and index,
tit. Oyer; 1 Chit. Pl. 369 to 375; Lawes on Civ. Pl. 96 to 101; 16 Vin. Ab.
157; Bac. Abr. Pleas, &c., I 12, n. 2; Arch. Civ. Pl. 185; 1 Sell. Pr. 260;
Doct. Pl, 344; Com. Dig. Pleader, P Abatement, I 22; 1 Blackf. R. 241, 3
Bouv. Inst. n. 2890.