Most modern literature refers to just OKRs (https://www.perdoo.com/the-ultimate-okr-guide/) but I prefer adding Goals to it as well – as Microsoft did back in the 90´s. You need to have defined a clear Goal first before Objectives makes sense. Anyway, the operational stuff is captured in the OKRs (without the G) so no need to split hair on semantics, both work.
The power of the OKR driven operation is explained well in (https://www.amazon.com/Measure-What-Matters-Google-Foundation/dp/0525536221). If you google “measure what matters” you find a lot of material, Youtube videos etc to give you a crash course. Though the examples in the book are very big companies, the method works well for startups as well. Actually they may be even more critical for a startup, as “what matters” is dependant on the stage of the J Curve you are at. As you make progress, “what matters” should change. The thing to drive your everyday activity is the Key Result. The name is a bit misleading, it should rather be “Key Activity”, but this is the standard term so we stick with it.