eyefortruth,
The Rambam in Mishna Torah is ambiguous, and he was misunderstood by some of his contemporaries. The Rambam clarifies his position in his Maamar on Techiyas Hameisim. Yes, the Rambam holds that Olam Haba refers to where we go when we die, but because we will, G-d willing, be brought back to life again, that trip to Olam Haba will not be everlasting.
The Rambam holds that the term “Olam Haba” doesn’t refer to Techiyas Hameisim, but he agrees that sometime in the future Techiyas Hameisim will occur. The people that come to life, however will eventually die again and go to Olam Haba. This second time in Olam Haba will go on indefinitely.
The Ramban uses the term “Olam Haba” to refer to techiyas hameisim and the post techiyas hameisim period (he uses other terms for where we go when we die). The Ramban also holds that the post techiyas hameisim period will go on indefinitely and there is no second time in a purely spiritual world.