おそらく

おそらく indicates high probability — roughly equivalent to "most likely" or "in all probability." More formal and literary than たぶん.

Comparison (PROBABILITY SCALE)

• きっと: surely, definitely (highest certainty)
• おそらく: probably (high probability, formal)
• たぶん: probably (moderate probability, casual)
• もしかしたら/もしかすると: perhaps, maybe (lower probability)

Position in sentence

Typically placed at the beginning of a clause or sentence. Often paired with だろう or でしょう.

Writing

Can be written as 恐(おそ)らく. The kanji 恐(おそ) means "fear" — the word originally implied apprehension about what might happen.

Register

Preferred in writing, news, and formal speech. In casual conversation, たぶん is more natural.