Although there is very little Old Cornish, very little of what exists is online. Heather Rose Jones has an online article "Cornish (and Other) Personal Names from the 10th Century Bodmin Manumissions". There are some notes about the Bodmin Manumissions (among others) at the Anglo-Saxon Charters website, though no actual text.
(And this is Middle/Late Cornish, rather than Old, but I thought I should mention that there are online facsimiles of the Beunans Ke manuscript and Beunans Meriasek manuscript.)
Jackson, Kenneth, Language and History in Early Britain: a Chronological Survey of the Brittonic Languages, First to Twelfth Century A.D. (Edinburgh: Edinburgh University Press, 1953; repr. Dublin: Four Courts Press, 2000).
Schlutter, Otto B., "Das Vocabularium Cornicum und seine beziehungen zu dem ae. vocabulare des XI. Jahrhundert aus Ms.Gott Julius All, 4°, föl. 120v17–130v", Anglia, Band 33, Heft nr. 3 (1910), pp. 370-390.
Vocabularium Cornicum, ed. by Johann Kaspar Zeuss, in Zeuss, Johann Kaspar, Grammatica celtica: e monumentis vetustis tam hibernicae linguae quam britannicae, dialecti cambricae, cornicae, armoricae, nec non e gallicae priscae reliquiis, 2 vols (Leipzig: Weidmann, 1853; repr. London: Routledge, 2002): 2nd vol.
Zeuss, Johann Kaspar, Grammatica celtica: e monumentis vetustis tam hibernicae linguae quam britannicae, dialecti cambricae, cornicae, armoricae, nec non e gallicae priscae reliquiis, 2 vols (Leipzig: Weidmann, 1853; repr. London: Routledge, 2002).