Ant. Many, together, several, mixed.
Onset, Onslaught. Assault, attack, storming, aggression, invasion.
Ant. Repulse, resistance, protection, shelter, support, defense.
Onward. Forward, in advance, ahead.
Ant. Aback, astern, backward.
Opaque. Impervious, not transparent, obscure, unintelligible.
Ant. Pellucid, translucent, transparent, obvious, clear, intelligible, perspicuous.
Open, v. Unclose, disclose, expose, explain, begin, commence, initiate.
Ant. Close, shut, cover, conceal; conclude, terminate; misinterpret, mystify.
Open, a. Unclosed, uncovered, unprotected, exposed, plain, obvious, evident, public, artless, candid, free, available, accessible, undisguised, ingenuous.