So, I try to avoid the idea of “all senior officers are department heads”, because I tend to find that it implies whole branching chains of command that we don’t necessarily see on the shows. Further, not all ships have the same assortment of senior staff roles - they’re assigned as the Captain sees fit based on their perceived needs and preferences for who they want as senior advisors.
That in mind:
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A Science Officer isn’t inherently the head of the ship’s Science department. Indeed, many ships don’t have a single fixed science department, but rather contain an assortment of scientific teams in different disciplines who come aboard and depart as required by the studies they’re doing (this seemed to be the case on the Enterprise-D). Rather a ship’s Science Officer, in the vein of Spock, T’Pol, Jadzia Dax, etc., is a bridge officer who serves as the Commanding Officer’s foremost scientific advisor, as well as handles scientific functions on the bridge. Klingon ships have a similar posting - we see Dax take up that role on the I.K.S. Rotarran briefly. Voyager does seem to have had a Science Officer as well, early on - Ensign Samantha Wildman serves that role in early seasons, taking the science station on the Bridge a few times, but she basically doesn’t appear in the later seasons, and Seven of Nine seems to cover similar ground narratively from that point on even if she doesn’t have a formal role on the ship.
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The Operations Manager serves a similar role to an extent - Data was written to serve the Science Officer role in story-terms, but was put in a gold uniform because he apparently didn’t look right in Sciences blue - but also seems to cover Bridge engineering functions (allowing the Chief Engineer to focus on running Main Engineering). Ensign Kim covers pretty much the same job - operating sensors and internal systems from the bridge, providing information to the Captain about weird swirly things in space, etc.
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Also worth noting that Data’s Operations Manager role on the Enterprise-D and -E is different from O’Brien’s role as Chief of Operations on DS9 - he’s functionally serving a Chief Engineer role (but there’s no “Chief Engineer” on a station as a station lacks engines), overseeing mechanical and technological matters for the station and any ships docked there. He’s not in charge of the Operations Division aboard the station, but is in charge of the station’s routine operations. The terms “Operations” and “Ops” get thrown around in a few different situations; Admiral Kirk was Chief of Starfleet Operations in The Motion Picture, but that was more “the Operations that Starfleet undertakes” as opposed to “Starfleet’s Operations Division”.
So, really, you don’t need both. Pick whichever one suits your game better, or take both and accept that there’ll be some overlap in their roles, because both roles are primarily “spout technical and scientific exposition to the Captain” and “scan things”.