But surely it would do no harm to let the child have one pretty dress--something like Diana Barry always wore. Mattew decided that he would give her one; that surely could not be objected to as an unwarranted putting in of his oar.
"oar" が櫂、オールの意味を持っている事は知っていますが、引用文の "putting in of his oar" は慣用句に違いありません。 辞書で調べます。
Dictionary.com: put in one's oar, to meddle; interfere: He put in his oar and was told to mind his own business.
American Herritage Dictionary: Interfere with something or insert one's opinion, as in I'll thank you not to put your oar in when we're discussing a private matter. This term, referring to helping to row a boat, was first recorded in Charles Coffey's 1731 play The Devil to Pay: "I will govern my own house without your putting in an oar."
日本語の諺「船頭多くして船山に登る」を連想しますが、意味は一寸違いますね。 干渉されれば結果が変になると言う意味では関係はありますが。