The Old Testament describes the ethics of a community. So without a community to apply Old Testament ethics to, one cannot follow the Old Testament. One cannot follow the Old Testament alone.
Therefore you must choose a religious community to associate with and support. At a minimum, you should attend their religious service on most weeks. Ideally you should attend more often and actively support the community with time and money. Tithing is described in Deuteronomy 14:22-29. If this religious community is your community, then you should apply Old Testament ethics to this community.
Even though this requirement is third, it is the most important requirement. Without a religious community to belong to, religion is meaningless. The purpose of religion is to provide a moral basis for a community. Christianity and Islam have enough in common with the Old Testament for a follower of the Old Testament to be part of one of these communities.
I gave my opinion of the religious options, but the final decision of which religion to pick is yours.
Of course the Old Testament describes another alternative, namely an entire community of Old Testament followers, the Israelites. But there are too few of us today, so this is not an option.
The religion of the Old Testament was the first religion to survive in exile. Previous to this, religions were tied to nations, and if the nation was lost then the religion was lost. Now we face an even greater challange, being so few in number that we cannot even form a community. Mikraite is an attempt to address this challenge by forming symbiotic relationships with other religions. We hope that this will provide a reasonable path in life for intelligent moral people.