It is likely that your ancestor went into a temporary grave containing several soldiers, and that there was nothing sufficiently distinguishing about him to permit identification at a later date.

During the Civil War, it was common practice for deceased soldiers to be buried on the battlefield, given the logistical problems associated with removing their bodies to proper cemeteries in a tactically active war theater. Certain officers were removed as a courtesy to their families, but enlisted personnel almost always went into the ground very near the place where they fell. They might be removed a short distance to be grouped with other bodies in a single mass grave, but burial details didn't haul bodies very far. With dozens, hundreds, or even thousands of rotting corpses to bury, the gravediggers took whatever approach seemed fastest and most efficient.

Friends of a dead soldier, typically other soldiers from his company, might bury him in an individual grave if they had the time or inclination, and then place over the grave a wooden marker with his name and unit inscribed. These markers were soon deteriorated by weather, though, and many were scattered by foraging animals or taken by local citizens or later soldiers for firewood. Foraging wild animals, incidentally, were especially destructive, digging up the relatively shallow graves to access the decaying carion that they smelled beneath the soil.

Late in the war and for several years after the war, the Union government embarked on an extensive relocation program in which battlefields were searched for Union remains. Probably a majority of battlefield graves were located and their deceased soldiers reinterred into newly created Federal cemeteries.

Soldiers were not issued dogtags by the Government in those days, but soldiers sometimes purchased name and unit medallions from sutlers and sewed these into the lining of their uniforms, in order that their bodies might be identified if they were killed. Also, some soldiers simply wrote their names into the inside of shirt collars and waistbands, saving themselves the cost of a medallion, but clothes buried with a body typically rotted beyond effective inspection before most reinterments occurred.

When the re-burial program was completed, the Army published a comprehensive list of the reinterments. An alphabetical index identified every Union soldier recovered from a battlefield and/or placed into any National Cemetery. I have a copy of the publication (US Quartermaster's Roll of Honor) and I looked up Oliver Bath without success. The only two soldiers named "Bath" in the index were Charles Bath of the 93rd New York, who died in 1864 and was buried on Long Island NY, and Augustus Bath from an unspecified NY regiment who was killed and buried in North Carolina, also in 1864. There were also two soldiers named John Bathe who were reinterred under the program.

Finally, remember that although the battle in which your ancestor was killed was a tactical draw, McClellan was in retreat mode at the time and was scurrying to get his army away from Robert E. Lee. Therefore, his army didn't stick around on the battlefield any longer than necessary after the fighting, and so the probability is high that your ancestor was initially buried by Confederate soldiers, Southern contractors hired specifically for burials, or very possibly, slaves.

The bottom line here is that Oliver Bath was probably buried very near where he fell, most likely within one or two days after his death, and that all or a portion of his remains were retrieved by the United States government a few years later, and subsequently placed in a grave marked "unknown."

As for where Oliver's reinterment grave and "unknown" marker might be, I fully expect it's the Glendale National Cemetery, located about a dozen miles southeast of Richmond, VA. After all, the cemetery's grounds are actually on a portion of your ancestor's final battlefield, so I can't imagine they'd have taken his bones anywhere else.

If you want to visit his grave, you'll need to walk past the approximately 960 Civil War "unknowns" interred at Glendale. You won't know which one of those 960 graves is his, but you'll almost certainly have walked within a few feet of whatever may still remain of him.