Online OKR software can help manage OKRs and incorporate them into your culture & operating cadence. However, for the first one or two quarters rolling OKRs can be done in a simple spreadsheet (like the one we link to below).
While a spreadsheet doesn't scale well, it will work to create & manage OKRs initially. Using a spreadsheet can ultimately help you select the right OKR tool that will work best for your needs when the time comes.