Insurance startup Oscar Health Insurance Corp. has a powerful new ally in its costly battle to win customers from entrenched insurance giants like UnitedHealth Group Inc. and Anthem Inc. Google Capital, the Internet company’s growth-equity fund, has invested $32.5 million in Oscar, the startup said in an interview. The deal values two-year-old Oscar at $1.75 billion, up from a valuation of $1.5 billion when it last took funding in April, said a person familiar with the transaction.