Is it possible to lose Onyx rank in Halo 5?

Solution 1:

As of right now, no. Once you reach Onyx, you will stay at Onyx. You will be able to see your pure CSR, which starts at 1500. If you lose matches while at Onyx, you will just go down to 1500. While at lower ranks, you can move between the rank (bronze, silver, gold, platnium, or diamond) you are in (from 1 to 6), but you cannot lose a full rank.

Example: You cannot go from Onyx 1500 to Diamond 6 or Gold 1 to Silver 6, etc., but you can go from Gold 2 to Gold 1.

Sources: I have lost a several matches at 1500, Rank explanations, another source

Eventually, there will be seasons that will reset your ranks at the beginning of each season.

Solution 2:

If you lose enough games, your rank will fall below onyx 1500. There are also now seasons that reset your rank every few months.