Search for an OMPT mock exam today and most results are the same document re-uploaded on different sites: a Sample of the OMPT-A Mock Exam — 8 questions over 3 pages, with space to write your answers by hand. It's free and it's a fair first look at the question style. Use it.
But a static PDF can't do the things that make a mock useful: no timer, no maths-entry practice (you'll type your answers on the real test, not write them), no on-screen-calculator constraint, no per-topic scoring — and it only samples the OMPT-A. The official omptest.org also sells its own practice modules and mock test; this one is simply free, full-length, and interactive.
Whatever materials you use, do at least one full-length, timed run before the real session. Pacing is a skill, and a PDF can't train it.