This trick won't work. The problem is the "intervening if clause" that is in Oath's triggered ability. With a triggered ability, the intervening if portion of the ability is checked both when the ability tries to trigger and also on resolution. Here, at the beginning of your upkeep, when Oath triggers, the game sees that both players have an equal number of creatures. Therefore it won't trigger and that will be it. Any changing in the number of creature after that will be meaningless in this context.
However, let's suppose your opponent had one more creature than you in play when Oath triggers. In response to the trigger, your opponent Swords to Plowshares one of his guys, equalizing the number of creatures. When the triggered ability resolves, nothing will happen.
404.3. A triggered ability may read “When/Whenever/At . . . , if [condition], [effect].” The ability checks for the stated condition to be true when the trigger event occurs. If it is, the ability triggers. On resolution, the ability rechecks the condition. If the condition isn’t true at either of those times, the ability does nothing. This rule is referred to as the “intervening ‘if’ clause” rule. Note that the word “if” has only its normal English meaning anywhere else in the text of a card; this rule only applies to an “if” that immediately follows a trigger condition.
