My main issue with this has nothing to do with balance. The other fighting styles are all flavoured around being proficient with certain tools of the trade. Most (Duelling, Great Weapon Fighting, Archery, etc.) revolve around weapon proficiency; Protection and Defence indicate proficiency with shields and armour, respectively. Defence was already the least flavourful option, but it did at least reflect mastery of fighting with armour on. Offence, flat, is... very dull. I can't picture what that "style" looks like. The mechanic you have lined up isn't broken (it's flat-out worse than Archery in a lot of case), but it's boring from a roleplay perspective.

What I'd like to know is: what does this fighting style look like? Duelling involves a lot of precision, while Great Weapon Fighting is about heavy blows, Archery involves careful aim, Protection involves leaping to the defence of allies, etc. Defence, I believe, reflects warriors who wear tougher armour than most. What does a fighter with the Offence style do differently from another fighter? Renaming it could go a long way towards this. As it is, this seems balanced enough, but very boring, to the extent that I'm not sure what niche it's trying to fill.