That depends on your definition of "safe."

There are no known acute interactions between antacids (Tums, Rolaids,
Mylanta) and Prilosec OTC, nor are there any known acute interactions
between H2-antagonists (Pepcid, Zantac, Tagamet) and Prilosec OTC.

In other words, you are highly unlikely to drop dead or even feel ill from
taking an occasional Tums or Zantac if you are also taking Prilosec OTC.

Long term, it may be a different story. There have been several articles
lately that have looked at acid-suppression therapy as a possible risk
factor for certain forms of pneumonia as well as one of the many factors
possibly contributing to the rise of C. diff infections.

Adding acid-reducer/neutralizer on top of acid-reducer/neutralizer may
increase any risk since you are further increasing the pH of the stomach
for longer periods of time. Of course, there are situations where use of
antacids and/or acid reducers is appropriate, but the better option by far
is to try to control it without any medications.

Have you been to a doctor? Chronic heartburn can be an indicator of more
serious problems, and untreated/mistreated, can lead to more serious
conditions.