脅かす・脅す

Here we have another ambiguous reading.  With the okurigana of kasu, this can be read odokasu or obiyakasu.

When the word in context means “to frighten/threaten (a person),” either reading is acceptable.  When the meaning is “to threaten (something other than a person)” — that is, when it used is abstractly — it is read obiyakasu. As a general rule, when what is either threatening or being threatened is not a person, then obiyakasu is used.

Consider these two examples:

熊もいたずらに人をおびやかすために出て来たのではない。

岡本綺堂、「半七捕物帳・熊の死骸」

…逆に勤労婦人の生活安定をおびやかすことにもなって来るのである。

宮本百合子、「いのちの使われかた」

When the meaning is “to surprise (a person),” the reading is odokasu.

As for the reading when the only okurigana is su, although there is an archaic obiyasu, it will in most cases be read odosu.