"Omniscient" means "all-knowing." Scripture declares that God's eyes run everywhere (Job 24:23; Psa 33:13-15, 139:13-16; Prov 15:3; Jer 16:17; Heb 4:13). He searches all hearts and observes everyone's ways (1 Sam 16:7; 1 Kings 8:39; 1 Chron 28:9; Psa 139:1-6, 23; Jer 17:10; Luke 16:15; Rom 8:27; Rev 2:23). In other words, he knows everything all of the time. He knows the future no less than the past and the present, and possible events that never happen no less than the actual events that do (1 Sam 23:9-13; 2 Kings 13:19; Psa 81:14-15; Isa 48:18-19). He does not have to acquire information about things; all his knowledge is immediately and directly before his mind. The authors of the Bible stood in awe of the capacity of God's mind (Psa 139:1-6; 147:5; Isa 40:13-14, 28; cf. Rom 11:33-36).
“Libertarianism is inconsistent, not only with God’s foreordination of all things, but also with his knowledge of future events.” John Frame (p. 143 The Doctrine of God),
“If [liberty of indifference] be the true theory of the will, God could not execute his decree without violating the liberty of the agent, and certain foreknowledge would be impossible.” A.A. Hodge (p. 210 Outlines of Theology)
“Moreover, not only are such contingencies not knowable to God, but also such ‘future, free contingencies’ do not and cannot even exist because they do not exist in God’s mind as an aspect of the universe whose every event he certainly decreed, creatively caused and completely and providentially governs.” Robert L. Reymond (p. 189 A New Systematic Theology Of The Christian Faith)