This largely comes down to personal preference. As long as you use an alignment method, like KPI Trees, it shouldn't matter to the ultimate outcome. If you do decide to do 'bottom up' OKRs, then you will still need to thoroughly understand your companies objectives and make sure that your low level OKRs connect with them in a meaningful way. If in doubt, go for top-down, as starting with the bigger picture makes alignment simpler and easier.