Short notation to indicate that a quantity is of order one without using an equals sign?

Question

Consider a plot in which one axis represents a variable from which a dimensionless number is formed, say a velocity, $u$, and the dimensionless number we are considering is the Reynolds number, $\mathrm{Re}$. I'd like to indicate the position on the axis for $u$ where $\mathrm{Re} = O(1)$ with say, a vertical line. Note that $u$ may vary over several orders of magnitude and may be represented on a log scale axis. I would like to highlight the significance of the line/point on the axis with a label, preferably a single symbol rather than a short expression. I'm looking for a shorthand for the $\mathrm{Re} = O(1)$ expression. What would be accepted here? Can I include a modifier symbol with $\mathrm{Re}$ to indicate that is approximately equal to unity / of order one? A subsript? A special font? Should I let a special (different) symbol be $\mathrm{Re}$ when equal to $O(1)$. This is a notation question, strictly speaking.