We don't see Ambrosia Beetle frass tubes, but this looks like slime flux or wetwood, a relatively common and often non-serious condition in older trees. If the tree is in decline, other factors are responsible and likely began many years ago, even if symptoms weren't obvious at the time. We'll share a few page links discussing tree decline if you're curious (particularly among oaks in suburbia), though a tree can be assessed by a certified arborist to better narrow-down possible causes. Arborists can't always intervene, though, and treatments won't resolve existing damage. They might be able to identify sources of ongoing tree stress, though, in case they can be alleviated.

• What Causes Trees and Shrubs to Die?
• Why Oak Trees are Declining or Dying
• Browning and Defoliation of White Oaks (Pin Oaks are in a different subgroup as White Oaks and aren't necessarily as vulnerable to the same stressors or diseases, but we include this link to illustrate similar issues among mature trees.)

Miri

Thanks!  Slime flux makes sense.  What appeared to be frass in the flux threw me off, but my eyes may have been playing tricks.

You're welcome.